Also FNAF World *was* on steam but was taken down by Scott for negative reception but can still be found on GameJolt.

Added for DD MM

My god, your artworks are so unbelievably gorgeous!

+rep

Amazing person with a great heart. I love you

Games I hope can be added on Steam someday...
(Their are probably reasons why they're not on Steam, but I'm writing them anyways.)

Mortal Kombat 9
House of the Dead series
Rare Replay (or at least Banjo-Kazooie, Banjo-Tooie & Conker's BFD)
FNAF World
Dragon's Crown

"PS2/GCN Classics"
Sonic Heroes
Shadow the Hedgehog
Pac-Man World 2
Tak and the Power of Juju

(there are others, I just can't think of any)

I'd also hope that they would have Trading Cards and Badges. If not those, then at least add Achievements to make the games more worth 100% completing.