I'm hoping and wishing for MP and JP backing vocals, both in the new album and live.
I have this unsourced, gut impression that JP is self-conscious about his own vocal abilities (given the tours where they went with him miming over a pre-recorded track, or the autotune effect applied to the released Wither demo), and that kind of sucks because he's actually got a very enjoyable voice with a lot of character. Solitary Shell wouldn't be what it is without his parts, for instance.
To have him AND MP doing the backing for James again would not only be a very cool part of the proceedings, but something we really haven't experienced in any capacity since MP left (by that I mean: we haven't been DT-less in that time, and both "sides" have played Dream Theater material... but I think the "gang is back together" sentiment will get even stronger when we get, say, JL, JP and MP singing the Fatal Tragedy chorus together or another part with that kind of effect).