As fun as it is to beat the dead horse that is Tsubomi and Ayumu's relationship, I kinda want to see other girlfriends she lost due to her siscon. That, and some growth on Tsu's side, she can't be bitter about it for ever, it's unhealthy.
On the other hand, it would be fun to see Tsubomi returning home to forget about Ayumu, just to see her with her sister/wife on their summer vacation.