For me, my number one is probably BioShock. The water effects already looked incredible 15 years ago, so I can only imagine how Rapture would look with 2023-onward technology.
Mercenaries: Playground of Destruction
One of my problematic faves. Better graphics, physics, destruction, longer view distance would add a lot. Also I think the Deck of 52 system could be improved and made more dynamic by taking inspiration from the Shadow of Mordor nemesis system (patented, I know).