I was thinking a B-17 Fly Fortress! All those gunner positions and the claustrophobic feeling!
And as long as we're dreaming, being able to play Co-Op within a single bomber, would be doooope.
Latency would be a major issue, but imagine if you were the pilot and me and 3 others were gunners on an intense bombing run. XD
But lets let them perfect this one first, cause it seems like they're willing to do a lot more (Even though they could probably stop here and be justified) :P