I've seen Pictures of a prototype 626 w/turbocharged engine and gobbs of power that may be the next MPS car, I have also heard that a MPV minivan MPS will be coming out with lots of toys.
Judging on how long it takes to design a car, I'm sure there were a lot of ideas flying around about what is going to happen in the future. I think mazda took it on the chin with the mp3 because of the need for more power. If you figure that mazda will do a MPS car or 2 for each model year, then say hypothetically that the 2002 year gets a MPV minivan and a 626MPS. Then in 2003 they could do 2 more, and also the RX8. I'm sure that some guy sat through a meeting and mazda played around with different ideas. Which ones will actually make it?? who knows.. probably not even Mazda...
I guess Time will tell.