Alright, I'll wait a little while. But I'll be very disappointed if I don't get to add this to the list because you gave up So you better make this mod!
Haha. Well, I'd like to keep my feet on solid ground. I know for a fact that most of the mods are never finished by a variety of reasons: from desinterest to simply lack of time. This is the first time I try to make a mod "for the public", so I'm just trying to take it cautiously. And considering that modding won't be possible for a long while, I can only get some ideas ready for when the moment comes. Oh, and meanwhile let you know.
The following is, obviously, a work in progress. I prefer not to think in the many times I'm going to be rewriting this stuff (that I have been rewriting a few times) until this is finished. 
Potential civilizations
- Bael Turath
- Tiefling empire named after the old Bael Turath empire of the ancients times. The tiefling warlock Bryseis recovered the ancient scrolls that contained the original pacts of the humans of Bael Turath and unlocked their magic to guarantee the survival of her people.
- Namaeth
- Cut off from the Feywild by the eladrin lords, the remaining of the elves struggled to survive until the elven druid Shava appeared and with her the earth was brought to life with each of her steps.
- Altesia
- The lands of Altesia had suffered greatly by the events of the Astral Wars and were filled with roaming beasts and bandits. A young human farmer named Alain heard the call of Bane of bringing peace to their lands and war to their enemies. The young cleric carved his place in Altesia in the blood and flesh of his enemies until he freed Altesia of internal threats and united it under his command and the worship of Bane.
- Dal Thormaer
- Expelled from the mountains after losing a series of wars against astral abominations and creatures of the Underdark, the dwarves had to look for a new home in the harsh and frozen surface. The dwarven warlord Baern united the remains of the clans and taught them of a new way, stablishing their new capital in the old ruins of a forgotten human kingdom.
- Drekhatar
- With the death of the Platinum Dragon and the last of the good dragons, the dragonborn felt hopeless and without purpose. With the end of the Astral Wars and shortly after the rumored raise of the new Bael Turath, the dragonborn paladin Heskan appeared bearing the mark of the Platinum Dragon and united them under his banner to fight what was to come.
- Ghezzra
- The peaceful people from Urden had a long history of abuses by other more powerful neighbors and the situation got worse when their lands became less affected than that of their neighbors by the bad weather. An unidentified avanger appeared and stirred the flames of hatred and vengenace on the normally peaceful people of Urden. He promised them retribution and no more abuses by their enemies. He promised the people of Urden that them and their bloodlines would survive. He promised them that all the unworthy would perish. And they killed for him.
- Kathra Daeth
- Exiled from her Kingdom, the dwarven Kathra founded her own clan with all those females that believed in her cause of gender equality. From the very beginning the amazons of Kathra Daeth have faced a slow extinction that they have accepted with stoicism. But the arrival of the dwarven Shaman Eldeth and the waves of exiles from the dwarven kingdoms have changed everything.
- Frozenheart
- When the blizzards ravaged his Kingdom, King Ruric commanded the performing of a ritual suggested by one of his counsellors and that would save the lifes of everybody in the kingdom. The ritual in reality was a necromancy ritual that turned every living being in Frozenheart into an undead being, from lowly zombie peasants to vampiric nobles. King Ruric still rules and athough he only cares about controlling his servants and keeping his lands as they are, not everybody agrees. Specially the vampires who need fresh blood to survive and have to resort to different means to obtain the food they need.
- The Red Tower
- This tower controlled by wizards has become a beacon of hope for many due to their ability to produce magical food resistant to frost. Their control of the sorrounding lands has increased quickly as refugees seek shelter under their protection and trade with other settlements allows them to get the few things they need. The living conditions are hard as slavery is favored greatly by the wizards, slaves being one of the things they trade their magical food for, but it's better than the conditions in wild.
Units
Levels
The units should have different level caps depending of the type. If it's a normal unit, the level cap should be 10. The Sovereign should have it at level 30. Heroes should be 30 too. Somehow I feel tempted of limiting Heroes to level 20 so the Sovereign really becomes the Epic character of the civilization but it'd not feel so heroic and not everybody would like to prefer to focus (only) in the Sovereign.
Powers
The normal units should get a basic power along with their initial class training. Each time they level up they should get a new power or bonus. The point that it's not clear at this moment is the Veteran and similar trainings that actually the units can get. Considering the nature of the D&D, for me it doesn't make sense to give the level 1 troops better stats keeping the level. Although it could be seen as better than usual characters (stats wise during character creation), seems more fitting to alter those trainings to make them alter the initial level of the troops along with the bonuses. For example, a veteran fighter could start at level 3 and an elite one at level 5.
To have some powers to be actually real attacks could be nice but they will just be stackable bonuses. So if he soldier has a power that gives him +2 to attack and according to the D&D books it's an "At Will" power, it'd stack with another one that gives +2 to damage and that according to the D&D books is a "Daily Exploit".
Different civilizations should get different powers for the same classes whenever it's possible. That should allow more differentation between them.
Equipment
Problem that I face actually is: what if I want to make archers? Fighters are usually seen as close combat damage dealers that they can be also archers. Rangers can choose between dual wielding or archery. Rogues, if included as an unit option, can be archers too.
If I choose Ranger as training for one of the elves of Namaeth, I could allow him to equip two swords for close combat and to be equipped with bows for ranged one. But what about powers? If he specializes in dual wielding, the powers shouldn't say anything about bows. Which means that I should control that once he pickes a power of one of those branches he doesn't get to choose again from the other branch, or that I should offer two Ranger training options and he would only get powers of the proper training. The last option I mentioned previously sounds better to me somehow but if I'm to include many classes later, it canbecome a very long list. Specially with Warlock pacts and similars.
In any case, as there are no special attacks of any type but stackable bonuses, it's not a big issue (I suppose). It'd be good to have special attacks though. After all, having my 100 Wizards casting Magic Missile from level 1 to level 10 is not so fun even if that Magic Missile scales in damage. Fortunate those Hero and Sovereign casters that surely will be able to have many spells by default.
Quests
Although events might be easier to create that new powers/skills for units and similar details, I think that this part is going to be quite difficult. The difficulty lies in several different areas, the most important one being to make them good from a reading point of view. At this point I think that quests should be one of the last things to do (ok, the very last one would be things like models for the damn Dragonborn or Tieflings... or Warforged? or Shifter? or Lizardfolk? or...?).
General Quests
Most quests should be general for any civilization and as fitting as possible to the theme of the setting. Hmmm Deadly and always with twisted ending?
Something in the lines of: "Go kill 10 winter rats.", you know.
Specific Quests
There should be some specific quests for each civilization. Especially important for the Master Quest. I won't know exactly which type of quests until the civilizations are fleshed properly but it's something to consider. About the Master Quest, I suppose that it shouldn't been too difficult to add more than one (tags or whatever). Although the basic Main Quest should be that or bringing the Sun to life again (as in the original concept of the campaign, supposing the PCs went the goody route), other types of Master Quests should be possible. Surely, undead would survive to the death of the sun (yeah, I don't like undead but a playable undead civilization is too cool to ignore) and if the Sovereign is too selfish to care about Earth, godhood can be a good solution for his problems (there is always the Feywild).
Those specific quests should have some level requirements associated to make sure that they only appear when there is always someone capable of completing them (Master Quest neededing most probably a level 30 Sovereign... although by itself I don't like that and would revise the idea later).
Consequences
Every quest must have a consequence. Some can be direct like "Get 100 Crowns", "Morale in your cities +10 during 10 turns" or "Inmune to 'rat Plague' during 20 turns". Others could offer (included as extra "reward") to unlock other new quests. Branching in the quest lines would be extremely interesting but has the usual problem of increasing the developing time (and stressing the brain!). Glad this is a mod.
But it could be cool if the evil Bryseis can end in the good side (or neutral) due to her choices during a quest chain.
WoE (World of Earth) 
Weather
The idea of Sovereigns changing the surface of the planet creates some interesting scenarios that were not in the original campaign (not that there were not lava terrain [Bael Turath] or plains/jungles but it was static terrain). It also adds difficulty to some of the original ideas. In any case, I should try the world to reflect what's going on.
A weather system would be excellent but due to the nature of the changing terrains, I don't see it happening. That said, snow terrain should deal damage to any unit in it each turn. Or that was one of the first ideas when traslating the campaign into this mod. There are pros and cons about it.
The world is not 24/7 covered in deadly blizzards. It's cold but there are also moments of calm that can last quite long. Unit receiving damage each turn they are in snow terrain would represent the dangers of cold and all that. Maybe some events that for a few turns increase that damage could represent bad weather. Some units would be naturally immune to that damage (or recieve less damage) for different reasons. Like dwarves being tough or White Dragons being in their element.
In the same way, lava terrain could have similar effects due to the heat, dehidratation or whatever. But although called lava, that terrain only seem to have rivers as such and the rest could have more normal temperatures?
Resources
Resources should consider the terrain too. Fertile Land patches that don't appear in snow but do so in plains is one example. Not sure of how easy could be and not sure if a snow based Sovereign conquers that city, what should happen with the tile: destroyed by the snow? It survives?
Another idea, that won't probably be appearing or that might need to appear only if it's made optional, is that some resources should deplete. Nothing adds to the feeling of end of the world like having your mines exhausted one by one, having less and less resources to survive with. Just my opinion. Access to other planes could prevent such thing but as we won't get them (by default)...
Other resources as forests (for wood) should be able to be increased (so elves can have lots of forests to play in).
Buildings
Not talking exactly about the types of buildings but about where you can place them.
So the dwarves of Dal Thormaer are exiled from their original territories but it doesn't mean they could not go back and build in the mountains again. Or the elves of Namaeth in the forests. Be it naturally and/or due to research, some civilizations should be able to build things where others cannot.
I'm going to suppose that buildings can be built in places "where the tags match" or something like that. That would make things easier, I suppose. Visually it could be weird though. I'm thinking of those underground cities in the mountains...
But if there were extra planes, underground cities would go in the underground plane leaving only one square in the surface to indicate the entrance to the city. Forest cities would remain unchanged. Aquatic cities would work like the underground ones, sharing map with them. But currently there are no plans of "aquatic adventures", especially if we consider that Stardock is going to focus on the land based action, which leaves the seas open to expansion (pun?) or mod. But I'd miss to be able to use somehow Kuo Toas and similars.