First thing first: tone down the overly elaborate models. I'd even say you should go back to sprites but they might actually be the more expensive option these days.
Factions: Haven (medieval human bumfucks), Necropolis, Inferno, Stronghold (orcs), Tower (wizards), Dungeon (subterranean monsters, no elves)
Bring back class change like in 4 but divorce it from skill gain, just give the option to pick a new class every X level so, for example, a Knight could become a Paladin (better casting) or a General (army buff).
Take three tiers of units from 6, you just have to have more roles for the units and give different factions different sets. So you could have in Haven a peasant (cheap, gives money), a pikeman (not too strong, retaliates before he is struck) and an archer (shoots) while Stronghold would have goblins (cheap weak shooter), warg riders (fast, more expensive) and orcs (tough, poor initiative).
Take initiative from 5, map size and shape from 3.