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Oh, it's been a while since I last posted off-topic outside the same... Let me describe my own ideal version. I can't promise that my dreams will align with yours.
Number of Factions: the more, the better, but without going overboard. Ideally—something between the third and fifth games. Mandatory factions: humans, mages, elves, dungeon, undead, demons, orcs (almost a set from two or four). Additional factions: swamp/lizards, nagas, dwarves, one or several new original factions as desired, although I'm afraid to count on the latter—what if they just make a second undead faction across the sea of Irsus.
Faction Concept: more individualism; don't be afraid to make factions without shooters/flyers/tanks/support mages. Unique faction features and abilities are strictly mandatory; all tactics should be built upon them. Let one faction overwhelm with cheap, numerous meat-shields, another be thick and impenetrable, a third rely on speed, a fourth flood the same from a distance with various nasties, units of the fifth help each other in different ways, and so on—the point is clear. Regarding design—you can follow the line of the sixth game; everything was pretty enough there, only the units need more individuality and a more diverse color palette without losing the unified faction style. Transports and little ships should also be unique; I really like the fifth for transports and the fourth for ships. You could also play around with the interface design; it was quite tasty in the second—separate designs for "good" and "evil" factions, but you can go even further here—let everything for elves be entwined with leaves, bones lying around for undead, caves, crystals, and little mushrooms for dungeon.
Heroes: graphics-wise, one should look to the fourth or sixth games. A unique model for each gender and each specialty is very cool. As for skills, I really like the system from the fourth; a few features from the fifth can be added as desired, like hybrid skills and other unique abilities. Oh, and of course, unique starting abilities—and make them truly unique, please. And remove this nonsense with the main starting hero and under-hero support units. Although the reputation system isn't the worst idea; it could be developed.
Magic: once again, rays of love for the fourth! Unique magic for each faction is incredibly cool. You could, of course, add something like "wisdom" to learn same little things from other schools, you can make "hybrid" spells like demonology, you can somehow combine all this with the magic of the fifth, or give one to three factions per school of magic. The most important thing—more spells, good and varied (the new King's Bounty gives a good example here), and a corresponding book, at least like in the first four games. And bring back a proper Mages' Guild.
Units: six to seven per faction. More is possible, but then it makes sense to introduce branches like in the fourth or Disciples. As for creature levels—four to five are enough so that the power gradation is obvious but not too extreme. Five to seven slots in an army are sufficient, depending on the number of creatures. Regarding upgrades, you can follow in the footsteps of the second: from zero up to two. It's better to take the unit window from the sixth—rotate them, examine them from all sides. And plenty and plenty of neutrals. I really like King's Bounty regarding unit windows—everything is very detailed, informative, and clear. Special rays of kindness to them for indicating resistance (not to be confused with defense) and damage type; I can't wait for such a feature in Heroes. In the sixth, they messed around with physical and magical attack/defense, but it still wasn't quite right, although you could develop that idea—it would even be better if it were something original rather than copied.
Castle: the second and third games have the most beautiful castles, but such images are no longer very relevant or attractive. I wouldn't refuse a version like the fifth—pretty 3D with a flying camera. Again, clickable buildings duplicated by buttons. The construction screen—the fifth and sixth were quite good; one can move in that same direction, only draw the constructing buildings larger; you can look at Disciples for this. The type of terrain on which the castle stands could influence the city screen, but I don't like the version from the fourth—everything there is too crude and the faction style is killed at the root; it needs to be implemented more elegantly. Unit recruitment was very good in the third and fourth—seeing all the guys at once with characteristics and miniature models. And as for resources, bring everything back to how it was. Again, faction individuality: some need sulfur, some stones, some something else.
Map: less beauty, more functionality; put some tasty landscapes around the edges, fewer corridors, more resources, more artifacts, more unique buildings and other treats. Seriously, the S-map in two-three-four was much tastier and richer than the XXL-map of the sixth. In terms of "treats," one should look to the third and especially the fourth. I understand that graphics require beauty and few people hunt through every corner now, but it must be interesting first and foremost. Give external dwellings to every unit. Kick territory capture and other forts back to their native Disciples. Caravans must definitely return. Cut down on castle rebuilding—let it take longer and let the whole city have to be rebuilt from scratch. You could remember squads without a hero, only let them not capture anything and let the battle happen in auto-combat mode. Oh yeah, remove those druggy map settings from the sixth; it's unclear why they disliked the "classic" starting choice of faction, color, hero, and bonus.
Battle: keep pushing what exists. Cover—good, unique objects with perks—good, unique unit abilities—good. I'd like to bring back the heroes from the fourth, but I predict a lot of spitting and anathema, so you can make them closer to the fifth. But let the hero stand on the battlefield both on foot and give a perk to nearby units, while the horse grazes somewhere aside. Keep all the beautiful crit animations as they were, but make them toggleable. Let the turn order be standard—each unit moves at least once per round, determined by initiative, etc., etc. And don't forget to put the hero in the line. Walls and gates should still only be destroyed by a catapult. War machines should be there; they are good, though skills for them should be merged into one. And separate rays of love to the sixth for the funny unit idle animations.
Additional: a couple of new original treats. Not too radical, but interesting. Play around with war machines, unit parameters, hero skills, the map, the size and shape of the map and battlefield, introduce three-way battles (at least for duels in Multiplayer), and so on.
Additional-2: approximate faction composition.
Humans: mostly various human paladins, spearmen, archers, monks, cavalry, swordsmen, and other joys. Angels can be added; I don't really want griffins, but they can be too if you must.
Mages: standard and beloved gargoyles, golems, djinn, titans, and other magical creatures of varying degrees of mythological own. One or two actual mages as desired.
Elves: fairies, archers, ents, and unicorns are strictly mandatory. Druids, pegasus riders, a few more elves, various walking cacti of different shapes and sizes, forest or magic dragons (you can make them small and put them at low levels), phoenixes, optional griffins, and various animals from wolves to my favorite giant praying mantises.
Dungeon: since the devs don't want to leave Ashan, dark elves will be there: assassins and witches, as well as their pet minotaurs, manticoras, beholders, and everyone's favorite same-old black ones. Optionally various faceless and a couple more squads of actual dark elves.
Undead: how could it be without skeletons, ghosts, and vampires? And there are also dark knights, liches, zombies, bone dragons, lamassu (I really liked those kitties) and a bunch of other spider-like decaying not-quite-living nasties.
Demons: here you can go wild! Imps, cerberus, and archdevils are strictly mandatory; incubi-succubi, spawners, pit lords, and other horned red own-evil nasties of all possible shapes and sizes as desired.
Orcs: goblins, cyclops, shamans of any gender, centaurs, harpies, a couple or three orc warriors, seasoned with thunderbirds, hippos, some bull riders, and a couple of monsters as desired (even those same griffins are fine—why not?).
Lizards (I know they aren't in Ashan, but I really, really loved the swamp in the third): several lizardmen—warriors, shamans, maybe shooters or other throwers, hydras, wyverns, basilisks, maybe a few dinosaurs, snakes, and other reptiloids.
Nagas: medusas, kensei, kirins, yuki-onnas, kappas. Additionally, one or two nagas squads, something mermaid-like, kraken, and various Japanese-style water undead.
Dwarves: bear riders (male/female), a couple or three dwarf squads, mechanical dragons, yeti. You could also have various mechanisms, giants, some Scandinavian-style freaks, and even valkyries.
New: a couple of human or non-human factions—some savages or bandits; style and habitat as desired. Something non-magical but with good technology—better than the dwarves. Some mystical unknown thing, dark or light. Any interesting humanoid mythological race with accompanying creatures. The main thing is to skillfully fit all this joy into the world's history.
In short: plenty, tasty, unique, with development and in a "heroic" way. And in the end, it turned out to be a hybrid of previous parts with the tastiest ideas from there. I'm afraid that is the fate of all players' hopes regarding a new part :)