I would love to see a Shotgun aswell, one of the most iconic weapons of the game. Although Id love to see an airsoft version more!
Why is it bungie decided to make such bizzare guns? The shotgun is top loading (very very odd), MA5C the magazine is too far back to even fire same with the BR55 and the M6 pistol is even worse, probably the biggest lunacy is the M7 caseless SMG, side loading? Not possible without some kind of 'bendy bullets' lol. I do love them though they are very cool.
As for the shotgun (again) I agree with previous ideas use an M3 Bellini and mod it with some time, money, work effort, fibre glass and bondo i think a relativly accurate Halo Shotgun could be made.
A fuel rod gun would be AMAZING. If someone could make one that fires 40mm shells from a magazine would be amazingly cool with out a doubt.