SteamU? Steam on the WiiU? Steam University?
https://twitter.com/SteamDB/status/826890027883982849
For those participating in Steam Client betas, make sure you updated and you can find these in the "clientui" folder in your Steam folder.
If you ask me, it probably stands for Steam Universe, which would fit into all the directions they are going in. Comics also makes a bit of sense now that I think of it
All those XBOX games means one thing:
The U is for UWP.
Steam UWP client, Valve + Microsoft partnership.
That is an interesting thought. Would not be pleased with that considering I'd want to be able to mess around with Steam and what it interacts with rather than what Valve / MS dictate