Imperium Galactica 1 & 2 - GT Interactive/Infogrames/Atari I assume the rights situation is complicated, because the first game is a DOS title, so that one at least wouldn't be held back by compatibility issues.
Theme Park World - EA I'm pretty sure, if this does have 3Dfx capabilities and can use nGlide, we'll see this show up in the coming months after Theme Park has sold well (unless it's a situation similar to SimCity 3000, and for whatever reason the next game in the series doesn't appear).
SimAnt - EA I wonder if GOG hasn't brought this because it might be viewed as "edutainment" (there is a DOS version, right?), a shame if so because the game seems to have sold really well and was well-received.
http://en.wikipedia.org/wiki/Sim_Ant#Sales_reception Cave Story+ - Nicalis Inc. As far as I can tell there's nothing really holding this back from a release here. Of course it's possible the publisher just wants to stick with Steam.
Worms World Party - Team 17 I assume the price wanted by Team 17 is holding back a release here, similar to Worms Armageddon, but maybe it's compatibility or multiplayer issues.
Battlefield 1942 - EA It's free on Origin to entice people to use it, afaik. As such I suspect it will be quite some time before that game shows up on GOG.
Turok 2: Seeds Of Evil - Acclaim Similar to the first Turok I suspect it could show up through Valiant or Night Dive.
Duke Nukem 3D: Duke It Out In DC - ? (Duke 3D expansion) If DD were to bring Megaton we could see this show up, but unless I'm mistaken it is technically free anyways.
Anno 1602 - Sunflowers (acquired by Ubisoft) Developed by Max Design, I was in contact with on the MD guys on Twitter. He seemed to suggest he couldn't bring the games because there were publishing issues, but if Ubisoft does still have rights, I'd assume we could see this show up in the near future. (Of course that's assuming no compatibility issues.)
Hitman: Blood Money - Squeenix Maybe it's a situation similar to Team 17 - Squeenix wants more money than GOG wants to sell the game for. They did recently release an HD version; maybe after that has had its time to sell, we'll see this one show up.
Mass Effect 2 - EA /Maybe/ next year will see more "new" EA titles released on GOG (the newest one so far has been Jade Empire I believe), depending on what EA wants to keep exclusive to Origin. Of course, the first one will have to come first.