I think the obvious choice for a rotation would be PLS-on. Keeping with the semi-consistent practice of rotating to the first set released the previous year. This would make the most sense as most of the cards in BCR are reprinted and Landorus EX would not be able to reap the rewards of all the fighting support coming out in Furious Fist (at least not in Standard).
The other option would be a rotation to BCR-on. This has some uses as well but is less likely to take place. The benefits would be keeping all Ace Specs and Skyla playable, also giving Landorus EX free reign to run rampant with all the fighting support. I really doubt this would be the rotation but I don't think Landorus EX is such a bad addition to the next format that this rotation is not worth considering
The final and most unconventional rotation I think might take place is to XY-on. Now hold your horses before you start going crazy, this could actually be a potential choice that may make a lot of sense. Considering the fact that we will have Day 2 tournaments in the Extended format this wouldn't be so detrimental to player's collections and it would create an interesting and dynamic Standard format. It would be much slower and very rewarding to skilled players. Regardless of how ludicrous it may sound, it's worth keeping this rotation in the back of our minds.