Command & Conquer or Age of Empires
Monkey Island.. but only if they made one in the style of Curse.
Tomb Raider.. but only if they made one in the style of Core Design (including tank controls).
Age of Empires.. but only if they stay true to the original formula.
Commandos, Deuteros and Lemmings.
Bioshock without a doubt.
Golden Sun
Ratchet&Clank
Jet Force Gemini
Timesplitters 4 made correctly would be the greatest thing in a long time.
Fable, but with them actually releasing the games for PC users as well, maybe through the Microsoft App Store
Croc: Legend of the Gobbos!
Summoner, easily the Summoner series. Preferably more in the vein of the first game as well, i preferred actually summoning my monsters as a new party member over BECOMING said monster.
Knights of the Old Republic, still play it to this day.
Also Baten Kaitos, most fun battle system of all time.
The C&C: Generals games were fantastic, I would love a new game.
The obvious one is also Half-Life. Loved every single release, even the HL1 expansions.
The SWAT games were also pretty cool