I honestly believe this would be the best combination ever. While there has been requests for bringing Steam games to the Xbox One (which would be easy, even if just streaming) I can't see Valve and Microsoft condoning this as well since Microsoft would prefer people buy Xbox games.
But there is no conflict of interest with the Wii U. Nintendo's library of games doesn't have as much overlap with the Steam Library, so you can literally get the best of both worlds on one console. I don't think it would be incredibly difficult to create the app for Wii U, since the most processor intensive would be the In-house streaming part. Nintendo would probably co-operate with Valve a lot for this, yet their (currently) limited market share won't kill off the SteamOS project.
Thoughts?