I think this is the winning move, going for the big sandbox games with strong modding scenes. Personally, I'd pick Neverwinter Nights 2 and GZDoom over Roblox and Portal 2.
Portal 2 (It's literally anything at this point: a web server, a game engine, a fucking computer, a lot is possible when the right people crack it open)
Has it's modding scene opened this much? I thought people were mostly making test chambers.
Honestly, no idea, lol. I never went beyond NWN 2's original campaign. I just figured it would have more potential than NWN 1, in a world with only 5 games.