Had a similar thought, got lost in a General Thread.
Basically make it about four pounds, meatier in the expense department, but pretty much pick up where the MP7 leaves off. Much bigger clips designed to chew things up at close range, take the head off a FP, finish a charging Scrake (not that you need to since Crossbow>Scrake for any perk), and hamburger a big group of ZEDs for an emergency.
Give it nastier recoil and damage output, with the dart launcher as a side.
Still though, as much as I'd love to see it, I don't think it'll happen, not unless they did something to the MP7 like gave it a three round burst instead of full auto for its main fire.
They'd be just too similar.