Keeping in mind that this is a single player game and it is sometimes fun to be godly...

I agree with pretty much everything you said.  A lot of it can be boiled down to % enhancements, even stacking, rather than straight #'s.

I haven't used destinies gift/paragon yet... I may need to play with it before it gets nerfed.

The volcano definitely should not instantly kill sovereigns (and thus entire factions) without the ability to recover at the nearest city, it's the only thing in the game that has this effect, it seems like an oversight.

The rest is more debatable - in general I'm okay with the idea of city destruction, spamming paragon/destiny's gift, etc.. the mana costs are not trivial, I feel like there should be some dramatic way to end a game once you've stockpiled enough mana, by the time you can burn 500 on a volcano or several stat-boosting spells it should be about time to conclude things anyway. I'd agree with higher mana costs (say double) on all those spells though, just to delay that dramatic endgame a little more, it can come too quickly. A range limit and/or cast time on the volcano would also be good, and the hp removal on paragon needs to work, that's definitely a bug.

I think most of the scaling works out well; linear isn't inherently bad, not every effect needs to be great both early game and later on. Obviously inspiration's +1 research sucks late game, but it's great early game, it has a certain niche that it fills well. Of course if the spell was say a flat +1 research and +5% per essence to the city's total, it could fill both niches, early and late game - that kind of universal usefulness is a good ideal to aim for, but I wouldn't call it essential. That being said there are a few stats where the linear scaling is just broken, dodge leaps to mind; diminishing returns would work well there.

Personally, I've never felt that this was worth ruining the magical power of the majority of my spell casters, since I find spells like Flame Dart and Fireball to be much more useful than low to moderate amounts of poison damage over ten turns and I don't like Cyndrum Demons. Plus, there isn't that much in Death Magic that shards are useful for, since its most useful spells (at least, the ones I find most useful) are curses whose effects are unaffected by shards, and I can get equivalent or better damage spells elsewhere. Certainly Wither is a nice army curse if I have a lot of shards, but Blind and Infection together are about as good, and Infection will spread any other curses I throw on the unit around to the rest of the enemy army, and both Blind and Infection have no dependency on Death Shards.

Granted, if I have enough high level conclaves Fire and Water magic can be largely unaffected by shard corruption, but that takes a while to come into play.

I'm fairly certain that a 5% bonus will remain fairly meaningless even in the endgame because of just how low the numbers for research are on any given city. If it were a bonus to gildar production, then it might be worthwhile on a town (though that is affected by the tax rate), but I suspect that research would be largely unaffected since research values are generally low even in conclaves. You might get an extra handful of research points out of it from your whole empire, at most. With the current research numbers, I think you gain more from switching to the +research per essence spell in the magic tree than you would gain from a +5% research bonus, assuming you're casting the spell on a site with at least two essence.

Regarding Paragon - I don't want the game to tell me how much health my sovereign must have before I can use this spell. If I want to make the trade-off that my Sovereign must sit at home because he's only got 1 permanent health because I had him cast Paragon multiple times on my heroes, then I should be able to do that. That my sovereign must have at least six permanent health to cast Paragon would be a reasonable restriction, but that the sovereign must have some arbitrary amount of permanent health that has no clear relation to preventing you from killing yourself? No thanks.

As far as the 'protection from strategic spells' enchantment/building/unit - maybe, but it'd need to have some kind of downside. The enchantment already does (sort of), since you'd be giving up one of a relatively limited number of essence slots for the protection, but the building doesn't have any clear downsides at the moment, since it's likely that the building would have no upkeep since none of the other buildings (that I know of) do. A unit that did this would seem somewhat strange to me - this seems more like something you'd have a dedicated group of people stationed permanently in the city to do, rather than having some traveling group that protects one city at a time, though I could see this for an army or as a temporary measure in a city.

With regards to rushing - it would be nice if rushed units had some kind of weakness or negative trait applied to them for being rushed, with the degree to which the trait is negative dependent on how far along in training the unit was when it was rushed to completion. I think it's rather ridiculous that I can rush a group of soldiers through training and have them come out with the same initial quality as it would have if it spent five or six turns training. Unless we're supposed to view rushed soldier groups as mercenaries - but then, shouldn't they have some sort of wage penalty (as in, I have to pay them more per turn than I would for troops I trained the normal way), and the mercenary rationale doesn't make much sense as an explanation for rush completion of partially trained units.

The Volcano has one other broken use, you can use it to seal off enemy sovs entirely.  Then just win the game.

Great feedback, thanks.

I'm looking at Curgen's Volcano (I think it needs a higher mana cost and a casting time) and Paragon's Gift (we need to fix the bug where it doesnt take hp if th sov is in a city) today.  I've already rebalanced Destiny's Insight.  Keep it coming if you discover other exploits.

First congrats on a great and addictive game.

Another area that needs looking at from a balance perspective is the stacking of different abilities and spells. I haven't checked this thoroughly but I believe that most abilities/spells stack, rather than just applying once. So for example if you have five heroes in a group and each has Tireless March, the entire group gets +5 to movement.  I believe it's the same with abilities that add to experience, defense, dodge etc.

As you can imagine, things can get easily 'broken' when stacked to extreme levels, i.e. a group of 9 heroeson horseback all with tireless march has a move of 13!