You might want to look at the Zoom RT-323; you can setup patterns to switch in all kinds of ways in realtime. Plus a LOT of other cpabilities (store patterns on flashcard, two drum tracks, a "song" track, and a bass track.)

Only gotcha is that it only has a 1/4 beat (true of all Zoom equipment), so no 7/8 etc. unless you do 7/4 and play at double tempo...

But the sounds are great, and the kits pretty extensive....