Spoiler: Sequel Trilogy
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The problem I have with Pete playing Rey is that there's a lot of characterization to get through, which is incompatible with how Pete plays the game. I think Rey pretty much has to be played by either Ben or Annie (or comedically badly by Jim) for this reason. Pete as R2-D2 works as well as it does because R2-D2 doesn't really have character as such - he's just the "plucky droid that could".

Kylo Ren might work, because he isn't around all that much in Force Awakens, especially early on. That gives Pete time to actually learn how to roleplay in time for the end of the movie and Last Jedi.

I think either him or Sally will be BB-8. Pete as BB-8 taking min-maxing to ridiculous extremes would be pretty great, Sally doing so to play the cute droid character similarly fits.

One thing I'm sure of - if/when they get to Solo, Sally will be L3-37. Or rather, she already is, because L3-37 was Sally's C3PO through-and-through.