I don't have much knowledge of guns but that looks like a beautiful weapon. I watched the video just to see them fire it. Having this in the game would be beyond amazing, especially if boss type zombies were ever added.
Yeah, I get this same itch late game. There are things I want to do like exploring interesting POIs that were previously too dangerous. The itch to start over gets pretty strong though.
I think one of the big things that would make loot more interesting is putting treasure rooms back to only being in the big POIs. This would bring back the risk/reward at least to some degree.