Start with Skyrim. Take that very streamlined, smooth RPG progression system and that quintessential Bethesda “pick any random place in the world and pick a direction and find an adventure” world design.
Fix the collision in the melee combat.
Add in some of good ol’ fashioned skill checks. If I have a 75 in Destruction, I should be able to skip finding the ingredients for this alchemist to make a battery and just charge it up myself. Let me Speech my way out of trouble. Etc.
Bring back some of the mystery of Oblivion. Instead of being like “Here, we dropped the factions right in your face so you can’t miss them,” let me go find them again. The Thieves Guild in Oblivion said “Okay, if you steal and you go to jail, when you wake up you’ll have a note from the Thieves Guild telling you when and where to meet. Or, if you get a beggar to like you enough (another mechanic I think they should bring back for these sorts of purposes), they’ll tell you about the Thieves Guild meeting. Or, if you’re in the right place at the right time, you can just stumble on it yourself.” I don’t mind that the Elder Scrolls games are a bit more amusement park ride than lived-in world (it means that I don’t get locked out of content that way), but it’s more fun to be able to find the ride yourself than get funneled into it.
Romance options! I miss BioWare.
Multiple options for finishing some quests.
I don’t ask for much, I know.