I was hoping for BCR-on, but the more I look at what's in Wild Blaze, the newly revealed promo cards, and what that format would look like, I'm going to have to say it'll likely be PLS-on. We're getting viable replacements for almost everything we lose: Computer Search's departure could give other Ace Specs a chance to shine. I predict Scramble Switch and Scoop Up Cyclone will become more popular in general, no matter what the rotation is. Sacred Ash is ALMOST Super Rod: Superior Energy Retrieval could fill the energy retrieval vacuum, or else some sort of Milotic/Iris combo will surface. Surprise Megaphone blows Tool Scrapper out of the water, so no worried there.
There is one other option that could be considered, not one anybody is really willing to discuss: the chance of a mid-season rotation. It happened before when we got into Heartgold/Soulsilver, thoroughly eliminating SP decks from the format and forcing players to rely on Evolution-based decks. I could see the same thing happening here and going to strictly XY-on, though that would mean letting Plasma run rampant until January or February. Again, this is just one of the many options TPCI has, and as they've done it before, it's worth considering.