For this I would add (if ever should KB tL go MP):
don't diminish to the factions only (like human castle, undead castle, orc castle) - but let people build their castles freely (make only heroes unique - and even more unique by free skill combinations)
Like you may start with necromancer hero, but you can select skills that will help your griffins be stronger together with your vamps
Hybrid armies is the BONUS of this game and throwing it away would move such MP game back in th pack of old HOMM series (not that HOMM is bad, but this hybridness of KB opens fully new ways of play)
By this one could battle his friend, where first one would be for Hybridness, the second one would be DWARF lover - the first one would benefit from the crazy combinations, but the second one from morale and dwarf specific skills/sets - who would be the victor?