I don't think Mike will join A7X permanently. There is the age difference (it was an issue when Charlie was significantly older than the rest of the band, don't see why that would change), the issue of how much control/pull he would have in an already established band, and how much time would he get away from them. I think (hope might be the more operative term) Mike will do a bunch of projects and eventually reunite with DT for one reunion tour/album.