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This is how I would like to see the seventh installment of the series.
Let's start with the factions. As mentioned before, the more, the better. Ideally, it would be something like this: Humans, Inferno, Necropolis, Magic, Elves, Orcs, Naga, Dungeon, Dwarves. You could also add something like elementals. They could be called Lords of the Elements or something similar.
Remove dynasties and dynastic weapons. IMO, heroes don't need that. Bring back the Mage Guild, because the system of 1 level - 1 spell is not very appealing. Add unique features for each faction. For example: For Elves (or any other faction), make a third upgrade for creatures of level 2 or 3. In the Mage Guild, let there be more spells, and so on.
Take the skills from the sixth game. You choose what you need, instead of selecting from what is offered.
A unique model for each hero, and specializations look good. If there is a random map generator, you could create a hero editor. Choose gender, model, class, and even skin color.
The heroes themselves should not participate in battles; they should just sit on their horse and cast spells and use special attacks once per turn.
You could experiment with special machines and come up with new ones. Walls and gates should only be destroyed by catapults.
The castles in the sixth game were generally good. 3D castles are not very good. They are difficult to click on. Of course, there are duplicate buttons, but I like to click on each building individually.
The number of creatures per faction should be the usual seven + upgraded ones. The unit windows with the "biography" from the sixth game should remain. And there should be more neutral creatures. Create external dwellings for all of them. You can keep the city rebuilding feature, because some people use it, and some don't. Forts are a useless idea. Squads without a hero can also be introduced based on the principle of city rebuilding. Bring back the old resources.
Approximate faction composition:
Humans: Various guards, spearmen, archers/crossbowmen, griffins, monks/vestals (I like the latter more), riders, and of course, angels.
Inferno: Imps, Cerberuses, Succubi, Devils, and something else.
Necropolis: Skeletons, Vampires, Ghosts, Lamassu, Black Riders, Liches.
Magic: Gargoyles, Titans, Rakshasas, Golems, Djinns, and just mages.
Elves: Dryads, Ents, Unicorns, Pegasuses, Elf Archers, Green Dragons.
Orcs: Centaurs, Furies, Orc Shamans, Cyclops, Goblins.
Naga: The same as in the sixth game.
Dungeon: Assassins, Manticores, Beholders, Minotaurs.
Dwarves: The set from the fifth game.
P.S. In general, the sixth game still needs to be developed and improved ;)