PT-on rotates 3 sets, when we already have a large number, and TPCi said they wanted to rotate 4 sets a year. I think SP has had its time, and RR-on would still leave their viability as techs in place - Luxray GL X and Blaziken FB X would undoubtedly still be popular - but a majority of the SP engine would be gone, leaving SP with very little support. This would make the most sense to me, just to get the monster SP decks out of the meta but still keep some of the most popular cards.
A mid-season rotation is probably not going to happen. However, if it does, I would actually think it fairly likely that it would be Black & White-on. As it is analogous to a "Base Set 3," this could become the one set Pokémon leaves in-format. Overall, I think it's very unlikely for this to happen, but only because of the unlikeliness of a mid-season rotation.