Aw, this is such a nice question! I have a horrible propensity to write very closed-off plots; the pirates AU, in particular, feels entirely done and dusted to me. I can, however, think of lots of small post-fic character moments or new-fic ideas for the Artist!Athos AU and the football AU (especially football - I’ve written so much of that that it could go on forever), and there are a few smaller possibilities for the Cold War AU.
I’m thinking - with agarthanguide's encouragement - of perhaps doing a snippet chapter where I could do Christmas in each verse, or something, later this year. But in the meantime, let's say: Artist!Athos and Muse!Aramis will fight and break up a hell of a lot, and be brought together again by Porthos luring them both to the restaurant at the same time and each time they'll pretend it's a surprise and fake their anger until they stumble upstairs. MI6!Treville will occasionally drive to Oxford and stop in at the cottage in the middle of Athos giving a local undergraduate a French tutorial, and when Porthos and Aramis get home (from being a porter at one of the colleges and being back in the Service) they'll see an extra dirty wineglass in the sink and Athos will just smile and shrug. Pirate!d'Artagnan will eventually collect some tattoos, but only if Athos and Aramis gleefully hold him down while Porthos works the needle. Constance thinks they're very dashing.