I love old school games, I love the trend of throwback styled games, pixel art, and I love metroidvanias.
So on that basis, my absolute dream of a game would be something that marries the gameplay style of DOOM Eternal; as far the movement and platforming mechanics. Especially the latter iterations we saw in DLC and horde mode (traversal sections) but with the exploration, interconnectedness, and replayability of something like Bloodstained: Ritual of the Night or Castlevania SOTN.
Already we do much of what metroidvanias involve in bite sized form via DOOM.
Just scale everything up. Imagine a gorgeous pixel art game with that same medieval fantasy style we saw recently in DOOM: The Dark Ages, but in a huge interconnected game world, and incentives to replay, take different routes, encounter different enemies, hidden bosses, bosses with multiple forms, etc etc. I would go absolutely nuts with joy.