My personal favourites that I would love to have updated are,
Frontier: Elite 2 (keep the same models, pyshics, maps and generaly everything else. However, juts update the graphics and sound. That would make the perfect game in my eyes.)
Detroit. The car manufacturing game. Make a car and try to sell it. Make updated model and flog that. Open new factories. Make more cars and make money. This is a must in my opinion. The ammount of racing games and car games in general that allow you to modify your ride and sell it on, online could really help out with the update of Detroit. All I would improve is the AI. I know thats agaisnt the OP's rules but hey, they are meant to be broken.