The more I think about it, the more I think that assuming that Portnoy's departure will mean less metal and more prog is misguided. Remember that Petrucci is as much of a metal fan as Portnoy is, although his roots seem to be more with classic metal, rather than some of the newer metal, so while they will likely lose the newer metal elements (like the occasional rough vocals), I suspect they will still be as heavy as they were before, generally speaking.
Also, I remember JP saying on one of the BC&SL youtube videos that he was tired of the long instrumental sections that featured guitar and key solos back to back, so perhaps we will see shorter, more focused, more concise instrumental sections?