Tonally, I do like the idea of a first/third person game where you are just one player, building and creating things, and NPCs move in and help/have their own agendas, all taking place over a period of time. Something like Unreal World, but where your actions effect the greater world; people use your roads, look into your land if you offer to sell it/make it communal/abandon it, etc...
But mostly, and this is a bit silly...
Graphically/atmospherically, I miss pastel colours and weird garish 80's fantasy. Where in Heroes of Might and Magic I/II, or Realmz/Spiderweb Software's earlier work, you felt like you could just randomly stumble across most anything. Crystals growing out of purple forests, vampires that made stupid 'bluh' noises, public-domain-y sound effects...
I feel like most fantasy tries to draw from the same popular-ish roots. World of Blizzardcrafthammer. And that's just fine, if those are things you like! But I have no interest in medieval power armour and ridiculous fantasy swords; I'd like to explore abandoned forts growing out of red-and-orange trees, look up at the alien sky, and feel a sense of peace. Perhaps it was because the market was oversaturated with 'weird' fantasy in the early 90s, in large part due to a lot of shareware. But I miss the environment, terribly.