Another awesome way for the Wasas to extend their powerbase - Anna Wasa was betrothed to the elector of Brandenburg, but then the elderly gentleman changed his mind and married Eleonore of Prussia instead. Now, if Anna were to marry the elector, it might remove the match between Gustav II Adolf and Marie Eleonore of Brandenburg from the table. The Carlsen and Johansen Vasa were in general at sixes-and-sevens, so even though Anna would be Maria Eleonora's step-grandmother, I could see the Swedes saying thanks but no thanks to the offer. The only other proposal for Gustav II I can find is to the Winter Queen, Elizabeth Stuart (but that would be unlikely for similar reasons - her mother was a Danish princess and Sweden and Denmark were at war). But it might mean that he marries elsewhere.
Then of course there's the matter of Anna Wasa's children (if any). She's older than most (35yo in 1603), but her mother was a year older when she married (36 in 1562), and was still popping out kids six years later (namely Anna), so I think there's a shot for one (maybe two) kids before Anna becomes a widow at age 40 (if her husband dies on schedule). Anna's kids would be next in line for the Polish throne should her brother's issue go extinct as OTL, but in the 1620s when those kids would marry no one can know that. The king of Poland might use them as proxies (especially a daughter, since he has none of his own until the late 1610s), or the elector of Brandenburg (Anna's stepson) might, depending on his relationship with his stepmother.