I would love a game that melds witcher style open world "ronin" with a tactics-style game like Fire Emblem. Sort of a wandering knight in a broader war sort of thing. The tactics-style battles you join (or don't) and the way they go matter to the line of main and side-quests and the condition or existence of characters you'd meet along the way. A different way to implement the whole "choices matter" mechanic that you get from Witcher.
I might go to a town, fail to take a side in the battle that town is locked in, and when I come back the town is laid waste. The quests that would be available to me in town are no longer there, but I might have errands where I end up searching the woods for refugees instead.
Something like that'd have a lot of replay value, because what you do changes the game you're playing over time.
there's some of that in Battle Brothers, with the player leading a band of mercenaries and taking part in turn-based battles and with an open-world map that can change (a little) as you play
I might go to a town, fail to take a side in the battle that town is locked in, and when I come back the town is laid waste. The quests that would be available to me in town are no longer there, but I might have errands where I end up searching the woods for refugees instead.
Something like that'd have a lot of replay value, because what you do changes the game you're playing over time.