A Unity-like or D implementation of Unity would be cool but really D on SteamOS needs 2 things:
1. stable engines for each part of a game-specific engine: I'm making a game right now that really can't be developped using Unity without totally fightining it so I use a combination of Ogre, RakNet etc. If I had a choice of composition of libraries to setup my game in D, I would have tried D. (even if the gc scares me frankly) 2. SteamWorks API compatibility: it's required for Steam, if I understand correctly, so you'll need to make sure it's possible to use SteamWorks library from D. I think it's all C but not sure.