I think the most important thing a new gun has to do is offer a new experience. The peacekeeper is an okay gun, but we already had stron SMGs and weak ARs, so it isn't unique or necessary in any way. It did not bring anything new to the table, and doesn't even have a unique attachment- if it were the only dual wield SMG then it might have been interesting, but no.
That being said, I'd like a new shotgun, as 4 is not enough for me, and I want DRAGONBREATH rounds. A pump-action flame thrower shotgun that does a little less damage up front than the 870 but continues to burn over the next few seconds would be different and add something we don't currently have. It needs to have a real range though, because as much as I love shotguns, the S-12 and M1216 are not worth using now that I have them diamond.
The mechanics of the dragonbreath would be similar to the spread of the 870, but since it is a cone the damage wouldn;t be spread out- it won't hit with one pellet at the outer range, it hits with more force. The reason being that the low initial damage means you are going to die more often to return fire, but allowing it a wide effective spread means you can actually clear rooms with it.
In BO1 I had a permanent class for both the Stakeout and the SPAS12-silenced, because they were both worth using. In this game I only have the 870. KSG is a slug gun, and doesn't really count, but it can be effective at least.
We could use a real magnum/python/mp412 (LOVED the 412) but they could fill that gap by overhauling the executioner to automattically have speed loader, as that slow load time is inexplicable punishment, and put the .45 rifle round as the final attachment. The thing that makes that gun so intersting is that it takes different kinds of rounds, and that the rifled barrel that causes the shot to spread so it has no range also allows for a bullet to be more accurate at range.
Pepper grenades, similar in effect to nova gas. Or mace or poison, but I think the reason we don't have Nova now is a ban on chemical weapons that the 60s didn't seem to mind so much.