I think devs will not add Saiga or AA12.
This type of weapon too expensive and not supposed to use in current combat situations. Anyway, shotguns are really specific weapon and mostly used in millitary as tool for opening shutted doors, dats why it belong to class "Breacher".
The most likely shotguns they can add to the game for variety are:
Benelli M4 Super 90 for Security
Semi-automatic, 5-7+1 capacity, build-in rail for mount different types of sights and possible polymer forend with few rails to mount attachments (FAB Defense BM-4 Benelli M4 Polymer 4 Rail System)
Franchi SPAS-12, stockless for Insurgents
Pump-action / semi-automatic, 5-8+1 capacity, can be equipped with rail (SPAS-12 / Elite / Prestige Mounts) for mount different types of sights and possible siderails for forend to mount attachments (like Magpul M-LOK MAG592 1913 Milspec Polymer Rail Section).
I dont really know can mercenaries or some kind insurgents afford this gun, but if they will - that would be really badass.