Personally I'd really like to see either a semi-automatic shotgun with 8-10 rounds for base capacity (Such as an M4 Semi-Automatic) or even a magazine-fed automatic shotgun (such as a USAS-12.) This would allow for 10 rounds in a magazine, which could be further expanded with a drum magazine. It would make for a great end game weapon for people who like to specialize in shotguns, and a good alternative to, say, the M-60, the trade-off of course being that they are more meant for close combat and become less and less useful at a distance. That, and the fact that such a level of firepower would encourage spraying ammo, which depending on the situation could be either really good, or really bad if you're playing on higher difficulties and looking to conserve your resources (or trying to avoid drastically raising the heatmap, for that matter.) The double barrel shotgun is cool and all, but in my limited use of it I find it to be a bit of a nuisance when you have several enemies charging at you. It's a good starter though, since at least in my experience it seems to spawn in gun safes rather frequently, giving easy access early game.
On a side note, I feel like the pistol tiers could use some work as well. The jump from the basic pistol to the magnum is nice and all, but its basically a side-grade due to the lower fire rate and the smaller ammo capacity. It would be nice to see maybe, say, a Desert Eagle which would give you a higher fire rate, and significantly increased base damage, making it more desirable for an endgame weapon, the trade-off being a very scarce ammo type and also expensive to craft, resource-wise, as well as significant recoil and heatmap advancement.