North Warriors is a great news
I would like to see a new multi-level campaign. What I mean is a single campaign as the ArmoredPrincess campaign or the first one, but with the option to start at different locations with different races and this would affect the quests, the story (in some degree) - like you are helping your race to win against the others in some world threatening events and decide the outcome.
I would like to see some skills affecting the creatures skills like in the first KB, but fixed (I remember that when you unlock some skill, it was unlocked for neutral enemies as well - maybe this was fixed with the latest patch but I haven't played that part so much as AP, so I cant tell).
I definitely don't want to see the micromanagement cut or simplified as some readers here suggested - see what happened to Heroes 6, it is a disaster and definitely a grand mistake, so please devs, keep this in mind, I already have thrown in the bin the latest spawn of a once great game (HMM) - not only for this reason but this counts as well. Leave the running around for buying creatures as it was. Don't make the game predictable and too easy and straightforward!
Create three-four destiny points in the main quests so you can have different game-end. Dont play with good-bad finals but something more interesting as pushing the world in this or that direction (and you can use this in an expansion as starting points, like a finale where the elves are enslaved and they fight vs. demons for freedom, or a world where the magic is banned, or a world with almost extinguished lizards, or a world which is too much order and no fun controlled by humans etc.)
As for the dragon/spirit/Valkyrie since it is only one spirit give it several directions for development, like stage 1, where you have to choose between might and magic, stage 2, where you have to choose between order and chaos - in this way you can deal with the longevity and the desire of the players to try different options - keep the spirit independent from the hero classes - three hero classes with order/might, chaos/might, order/magic and chaos/magic spirit gives you many choices and play-styles. I remember how fun it was playing with 4 different spirits. And you can do this with only 2 variables and a little imagination
Invent at least one more resource for buying creatures, possible 4. If you use one, you can earn it after fighting level 5 creatures to buy level 5 creatures and from main quests. When the player is out of it, s/he has to play again with 1-4 level creatures and again try different play-styles, not using only dragons for example. If there are 4 main resources, they could be used for creatures from different races and again you are giving more options.
In the first KB I liked the idea of gaining different noble titles but this had no effect on the game. You can use this in a way desired from many players here, and this is the idea for OWN castle, which the hero can rule and build. Each title gained gives you the opportunity to upgrade it and built some dwelling. I don't know if this is not in collision with the copyright and if you are allowed to move so closely to Heroes of Might and Magic, but if there is no problem, you can implement this idea. Each title will give you the right to build a dwelling of the next level, mage guild with researched spells, teleport to the castle, marketplace/artifact merchant. There could be side-quests like random notice from the game that the castle is attacked and you have to return to it, for a random fight against a hero/army.
What about creatures items? Every stack could equip one item and it alone would be effected by its special properties, not the whole army. Or you could equip the stacks with one potion to be used on that stack in the battle - only one again.
There are so many other ways to improve the game. You can fix Necro for example. What I mean is that it lacks (at the moment) the sinergy between units. Actually many other races lack this, but in Necro this is very obvious. Skellies are OK, Zombies - not, I dont think someone uses them in their own army, the two ghosts are pain in the ass when oversizing, vampires are strange with no life drain in human form and as bats they are not interesting (atleast for me) - play a little bit with numbers giving vampires life drain, remove the no-retaliation and instead of that give them unlimited retaliation - I tried this in a self-created mod and the results were very satisfiting both playing with and against them, give the Liches cloud attack properties to heal adj. friendly undead (they are supposed to do that and to help other lesser undead) etc. Remove the oversizing possibility for ghosts and because they are spectral units, give them chilling attack which makes the blood of all living creatures freeze or because they inflict fear-effect on their minds, next attack in the same turn from other undead is more powerful - this is one way to create synergy between units, or make all zombies taunt enemy attacks in some distance and give the liches the ability to resurrect into no-life all lvl1 and lvl2 undead (skellis and zombies), again you have synergy here.
And all lvl 5 units could have some auras depending on their aligment. This is another option to improve the tactic side on the battlefield. These auras could be improved by some hero-skills...