I’d wager that more integrated storage is the direction they’ll go. Flat kit inside the frame, hydration bladder, etc.
My money would be on revisions for the revised UCI rules. The biggest changes would probably be deeper fork legs and a deeper head tube to take advantage of the 9:1 ratio.
They could upgrade the P5 with minor changes, just like they did with the S5. If that is the case, my guess is 3-4 watts max. A clean sheet of paper redesign is possible, given that the last iteration came out in 2017 or 2018. That might look very different (wide fork legs and room for much bigger tires). My guess is that they won’t make it super chunky, given the technical nature of World Tour TTs these days I would bet on them also trying to keep the weight low like the Shiv.