One thing that would be nice is something like the Gamehub thing, but multiplatform.
I suppose that interfaces like KODI or EmulationStation could be used for this purpose, them are both pretty configurable for many uses.
The main issues, excluding DRM things (that are the worse), are the multiplayer protocols needed by the different stores... I'm mainly a single player, but I tried some games while online, and mainly STEAM games have most of the user base there, where in comparison the same games on GOG have the servers empty... and there's no way (as far as I know) to make a GOG game play in the same servers of the same game on STEAM...
If there was some kind of standard protocol and shared servers between all stores, then the situation could be much better...
I find GOG Galaxy pretty useful just as a launcher and updater, also because it's not so invasive... you can install a game through the client, then uninstall the client, and the game still works by itself (without anyway online capabilities), but stil the client lacks some functionalities that STEAM already have, for example to ADD external games not related to the store.
So, I can add all my GOG games to the STEAM client, but I can't do the reverse thing (also because DRM in this case)
The GOG-Connect thing, where you can get for free the GOG games you already have on STEAM, become useful there... but still there's the multiplayer issue...