Just did a GOG survey that focused on the idea of a paid membership option on GOG. Seems they're determining what people would be willing to pay extra for. Some of the options were
- a tool for backing up offline installers
- ability to install previous versions of a game
- extra insight into the preservation work they're doing.
- voting rights on games to bring into the preservation program.
And others that I can't remember.
I think the only way they can introduce a subscription without backlash is if they make it a purely community thing with a few bonuses. Give people access to special insights into their preservation efforts, special interviews, voting rights, Q&A, occasional free game, etc. If they lock features behind this like more cloud storage, or other stuff that customers simply expect with their game purchase, the press will be all negative.
It should be like Xbox Live Gold or PlayStation Plus. Some free games and lots of extra perks that you mentioned.