Even Wilson did decide to get PT back together, I wonder if Gavin Harrison would toss the Pineapple Thief in the trash heap to return to PT if the schedules conflicted. I suspect Porcupine Tree would be a better paying gig by a good distance, but it could be a one and done deal if Wilson decides to do one new album and tour and then disband it again, and Harrison may not want to burn the other bridge.
I honestly could see Steven bringing in a new drummer for Porcupine Tree. After all, Gavin replaced Chris Maitland, so if any one position was changed, I think changing drummers might not be a horrible thing. Maybe he'd get Chris back? That would be quite the move, but I would imagine that might also influence the sound a bit. I think if Steven and Richard have talked about it, they're both definitely in and I'm sure Colin would be as well.
Of course, there are plenty of choices for drummers outside of Gavin and Chris - Marco Minnemann would be an obvious choice, but he seems pretty busy with his own stuff and with Randy McStine. There's also Craig Blundell, but it also seems like he is always busy with stuff as well (especially since bowing out of Frost*). I could hear Thomas Lang drumming for PT, I think his style could be a good fit for them.
No matter what, though, whoever sits on the drum throne for any kind of PT reunion, there will be disappointed fans, especially if it isn't Gavin Harrison.
-Marc.