I wish there were some way to get Satch off G3. I'm sick of his terrible tone, I'm sick of his songs that don't change progressions at all, and I'm sick of him bailing out of runs with high pitched whammy bends.
While I'm at it getting rid of Satch, I think Paul Gilbert should take over his permanent position on the tour. PG seriously tore Satch and JP to ribbons on their tour - it was borderline embarrassing for the other two (and I love JP, but Paul is just leagues beyond in improv). Seeing PG and Bruce play Scarrified live was better than Satch and Petrucci's entire sets combined.
That being said, I think MJR would have to write some material first that was very guitar based to actually have viable material to perform at G3. The Dark Chapter is cool but MJR has developed so much since that material was written. I think guys who write in the style of Vai and Eric Johnson - that is, fantastic chords and chord progressions with constantly evolving leads with tons of attention to detail. MJR doesn't really have much material right now with that vibe, and a set of power chord based tunes gets really monotonous (JPs material is not varied enough to sustain an entire guitar based set). However, I think Mike is an extremely capable guitarist with tons of depth to his musical prowess and if he found the time to write some guitar based material, could fit in nicely on a G3 tour.