I love the ambiguity of the "producer" role. It changes so much between musical genres and even within them.
There's no real law about how much a producer is responsible for the creative direction of an album. Sometimes the producer is there to drive the making of the album through and "glue" things together (perhaps by improving/adding details to arrangements and supervising any of the mixing or mastering stages), and sometimes they're actually the fire under the artists' bottom -- having a songwriter's say on how the compositions go (perhaps even to the level of "how much each section is played", or in what order, or even the notes in the melodies proper).
Take the two latest Imperial Triumphant albums, where Trey Spruance from Mr. Bungle is credited as the producer. I've got an interview in the can with him about it, but to summarize, he didn't have anything to do with the songwriting -- all the sections and melodies arrived at his desk in a finished state and his main concern was adding instrumentation and effects (bas-relief, as he called it) and later mastering the whole thing according to his vision of how the final product should sound like.
Rick Rubin's productions with, say, Metallica or Slipknot (or any other bands, I'm just going from memory) are examples of the latter approach, where the ideas weren't exactly finished (even if they were demoed beforehand) and he'd guide the group creatively to arrive at the final tracks. (as far as I know, the Metallica guys were cool with it and some guys in Slipknot were too, whereas Corey Taylor was very unhappy with the process).
Anything is possible, but from what we've known of this band over the years, I do not see them feeling the need for an outside producer on either of those fronts. Any of those roles I mentioned are pretty much JP's and MP's bread and butter. I'm not inside their heads, but I feel this new album is as momentous to them as it is to the fans (if not more so) and, given that the albums in the Mangini era were at the very least VERY competent, they may feel like they have something to prove. Something like "we have to blow people away with this new album, because the others were already great".
I don't see an outside producer fitting this situation any more than Phil Towle coming in as band therapist.
Of course, I could be dead wrong and this is exactly what they're angling for (calling someone from outside the band to make sure the new album is tight and has a solid vision), but it seems ludicrous to imagine it'd be someone like Steven Wilson or anyone who'd want to butt heads with them in the studio. This isn't their first rodeo
Just a little reflection on the outside producer thing and why I don't really see it happening (and again, I might be wrong!).