I know it would never happen, mind you. But just think. The man behind Mr. Bungle, Fantomas, Peeping Tom. The voice of Faith No More. His creative influence seems to bring his very distinct style. After he began vocal work with Faith No More, many ans considered it to sound very similar to Mr. Bungle, despite the fact that he actually did very little writing on the album. Most things he's involved in just seem to end up with that distinct Mike Patton touch without actually sounding the same.
Combine this with Dream Theater. Their collective creativity tends to creative a specific sound, again without it all sounding the same. I'm thinking a mix of two such creative forces would be pretty interesting.
Then again, their style are different enough there's the chance it might turn into something...not particularly good.
Regardless, I know it would never happen. But alas, a man can dream...