...something like...THIS?
Just an example here...this is built into a $200 powered Zissou Pulplogic case, but I have to admit that I'm nervous about the power draw in this, as my spec came out to 324 mA on the +12 rail, and the cab's maxes are 350 mA on each rail. But the point is, it's possible to concoct a tiny little case strictly for drums and just sequence the whole thing off of a BeatStep Pro. Like that sequencer, there's eight drum modules here, although you'll likely want to not use one along with the others at any given time because the HATS909 module needs triggers for 'open' and 'closed' to get the sound right.
But yeah...stereo mixer + a 4-1 mono mixer for all your mono/center sources like kick, etc, drums, a mult to split a trigger to double a drum pair, and a stereo out. Very simplistic and small, but also pretty powerful. Stepping up to, say, 60 hp would probably yield more room, but I opted for the Zissou because it's super-portable for the example.