I'll start us off:
Clip at the front of a singled kinda breech(like Ice's LS one on NerfHaven)
Double NiteFinder Plunger Tube/2 NiteFinders into One Barrel!
I know this has already been done on NerfHaven but it is pretty tricky! CAN YOU IMAGINE THE POWER THOUGH!!! It would go miles!!!
Either a Manta/Double NiteFinder but with the barrels joined somehow!!
Or somehow putting 2 plunger tubes together a making another plunger rod on a larger scale and putiing it in some other gun's shell because it would not fit inro a NiteFinder!
Larger Scale NiteFinder
This one would be very hard, very time consuming and very expensive but if you got some 50mm PVC (or any size) and used that as the plunger tube then you needed a plunger rod which would be fairly easy to make, a MASSIVE spring, big O-rings and some big to small couplers for the barrel and some more smaller PVC. But the shell would be very tricky to make but because you wouldn't have too many pieces then it wouldn't have to be as big!
Sorry that everything is NiteFinder, there the easiest to mod. But hopefully you guys will put some on with different guns for the rest of us to try!!
Happy Modding from these ideas and others to come!!