I'm a huge DT and Petrucci fan, but this whole sequence of events makes me think that this stuff between Portnoy and the rest of the band had been brewing for a long time. I'm not too familiar with some of the candidates, but Mike Mangini has been mentioned more often than the rest.

They'll likely continue on with this new line up for an album or two, and then they'll do their rock-band thing and have some sort of reunion tour with Portnoy again. Generally the way stuff like this works. It'll be interesting to see what the new album sounds like without Portnoy though.