Advice for the rest of the campaign? Sure, but the last time I played it was in version 1.0, so there may have been some balance changes since then. Also note that several of these are spoilers (the ones I consider to be significant spoilers, I will black out as above - highlight them if you want to read them).
1. Money, resources, and research carry over from one scenario to the next.
2. Most of the time, heroes and their equipment carry over from one scenario to the next.
3. Experience, levels, and earned traits carry over to subsequent scenarios.
Because of (1) and (3) above, you might as well spend the time to search the map and make certain you've killed anything you can, and completed any available side quests (assuming you're into maxing out hero levels, anyways), because that will cause you to have relatively strong heroes and a relatively large bank at the start of the next scenario.
Champions who die (faint? pass out?) during a battle revive in your hero party if you win the battle, rather than at your capitol, but they don't gain any experience for the battle. Try not to lose anyone.
There is an experience penalty for having multiple champions in a single party. It might be advisable to have one or more of your champions take the traits that boost army experience gain, and Potential I/II/III and Knowledge are good choices at hero level-up. These might not immediately boost your champions' power like some of the other traits you might be offered, but they will counter-act the experience penalty, which can really hurt champion leveling with more than a couple champions in the party.
Specialize your champions, at least in their early levels. This is very important, as you may not be able to get decent or good equipment for all of them, and you almost certainly won't have the mana income for more than one (or maybe two, if you're careful) dedicated casters. You probably also won't be able to apply strategic level blessings to them, as you won't have the mana for it. If you do want to make champions into supermen loaded with blessings, pick one champion to do it to, and make him/her your primary champion. And know that if you create a superchampion, you probably won't be able to do any significant amount of spellcasting afterwards - mana is severely limited in the campaign, even if you make your scenario city a conclave with an oracle.
On champion specialization - pick two for melee tanks, and get them either Path of the Warrior or Path of the Defender (preferably both, if you can do it, but that isn't necessary). Pick one to be your primary caster (I suggest Relias, as he'll level most rapidly out of everyone you get, and Fire is a great school of magic for a damage caster), and give everyone else a bow or a magic staff when you can afford to. You really shouldn't be risking large parts of your hero party in melee combat, as there's too much of a risk that one of them will get knocked out, which costs you a lot of experience - the hero who is knocked out doesn't receive any experience for the battle, but because he was present everyone else gets the experience penalty as if he did share in the experience. I suggest taking Path of the Assassin or Path of the Warrior (or both) for the champions who get ranged weapons - Path of the Assassin is marginally better for champions with bows, as there is a set of armor-bypassing traits available, while magic staves already ignore armor.
Cities probably won't ever reach beyond the second level of development. Therefore, choose to make the cities into towns on reaching level two - in my opinion, towns provide the greatest benefit in the campaign as you can get a fair amount of additional money production from a level two town, your research options are both highly limited and very expensive (once you get past the first two or three techs, anyways), and a level two Fortress doesn't give you much.
The champion (Gnarri?) who makes you an offer when you reach the intersection just before the Tyundii shrine is mutually exclusive with at least one other champion that I can remember. That other champion can be found in the first scenario, southwest of where the ogres are. Take a look at both of them, and see which one you like better before you accept either one into your party (the second guy, if I remember correctly, will join you at a later point in time than the first guy, but I personally prefer the second).
Remember to buy several sets of decent equipment - champions who join you may not have a full suit of armor, or a suitable weapon to cover weaknesses in your party (or fill the role you consider them most suited for), and you might find them very far from a city that sells the reasonably-priced equipment. Remember that, unless you want to spend a very long time accumulating money, decent equipment is a full suit of leather armor, a mid-tier weapon (short- or broad-swords, boar spears, battle axes, first or second tier bows, first tier magic staves), a mount, and maybe a couple accessories (Ring of Embers, Belt of Initiative, things like that). If you already have your melee tanks, and are confident in your ability to keep your casters and archers behind the melee line, you can drop the armor and perhaps the accessories, but don't drop the mounts - and when you can afford it, buy the armor anyways. Chain armor is available at the neutral city in the first scenario; you might as well pick one or two champions to serve as melee tanks and get them at least part of a full suit of chain armor. If you don't mind taking a long walk back to your capitol or a neutral city, this doesn't matter so much - but the selection sold by neutral cities can be somewhat limited and expensive, and it's usually a much further walk back to the capitol than to a neutral city. In my opinion, you should prioritize getting a decent weapon, then a mount, then decent armor, then accessories - but if you want everyone to be a melee champion, prioritize armor above everything else, and consider investing in chain for everyone.
Carry a couple of the injury removal potions around with you - you'll probably have a battle or two go poorly, and removing injuries is important to maintaining the fighting effectiveness of your hero party, especially since you cannot remove heroes from the party (in fact, you may eventually have enough heroes that you cannot add trained units to the party).
Tyundii makes a great melee tank, but you don't get to keep him very long. Make the best use of him you can while he lasts.
There are several parties of Urxen (or maybe it was Trog) warriors spread around most maps. You can fight them for a bit of experience and a reward; I don't think that there is a downside to this unless you lose a hero during the fighting (or lose a battle).
If you go up into the eastern section of the icy area to the North, you can find a very nice sword and a free piece of armor (a chain shirt, I think).
Before entering the temple at the end of the first scenario, strip the lady champion you started with of anything that remotely resembles a useful (or valuable) piece of equipment. Or not, your choice.
