If I could design my own dream adult game, I'd have to go with something kinda like CoC mixed with Skyrim, magic, corruption, leveling, stats, followers, transformation, take all of that, and add in a bunch of other tags, Loli, incest, management, beast, so on so forth, all with built in mod support and all the tools modders would need, maybe text based, maybe 3d open world, both have charm, I think it would be interesting to be some sort of ruler of the land whose army has been wiped out, so the only choice is to collect a harem of different fantasy races to breed an army, I'm certain something similar has been done in other adult games but I can't think of any off the top of my head,
As for a non-adult game, fantasy based Earth Defense Force, instead of guns, rockets, missiles and mechs, you'd have magic swords, spells, wands, staffs, elemental bows and summons, instead of giant insects it would be hordes of goblins, orcs, trolls etc etc, add in heavily customizable MC, multiplayer, raid bosses, cataclysmic level finishing moves or super weapons to unlock on the highest difficulty, leveling, once you hit max level unlock some sort of paragon infinite leveling system, large destructible castles and villages, open fields, summoned mounts to cross the battlefields, all sorts of things.