Dray2k: Hell, you can make an entire metagame around gaining Galaxy levels or something like with Steam. Or do something that Valve didn't came up with. We can only speculate how things will fare.
3D printable designs (e.g. Figurine of Geralt, or miniature of that Spider Mech on Wasteland 2 cover)!
They could be rewards for Galaxy levels or simply created by developers to be traded between players like the Steam Tarding Cards.
GoG could even have one of their own artists do "Personalised Avatars" for a price, a true vanity item - money that would help produce a better Galaxy UI.
I know people say that very few own a 3D printer atm but I expect that will change, a few years from now they'll probably be part of a Multi-Function printer.
Maybe they could have a "physical shop" as well, so we could buy shirts, hats, White Wolf Pendant, miniature Cyberpunk figures, Letho's armour, herb pouch, Orins, etc