I'd take the roguelike as a basis (games like nethack and adom), but take it to triple-A polish and then hugely expand it.
At the core of the game would lie a huge library of different scripts and building blocks, that the game can stitch together in almost infinite ways.
Roguelikes typically on randomize dungeon layouts and small stuff like the mobs and potion colors.
In this game there will be nothing the player can take for granted. There would be a save function (unlike most roguelikes), but reloading would randomize everything you had seen beyond the save point, including monsters and NPCs, their weaknesses, the lore etc.
Discover a vampire and get killed just after you figure out it's vulnerable to silver? If you reload back to a point before you met any, it may be that it's only vulnerable to fire.
Plot would not only branch due to player actions, like some RPGs, but also branch due to the randomizer.
Metagaming would be completely eliminated this way.
That means high attrition game models where resource management is important, function properly and improve the game and atmosphere.
Weak mobs can be followed by unexpectedly powerful ambushes and reloading won't kill all the challenge and tension while exploring, because it will be different.
Oh and full party control of 6 members; a huge dynamic world with wars and shifting borders (a la mount&blade); a player controlled stronghold; crafting; etc.
The only thing I'd keep to a minimum is dialogue, because too much chatter about NPC childhood traumas and therapy sessions, bores me. What is there would still be top notch voice acting ofcourse.
You asked.
