The generic choice is an underbarrel grenade launcher/shotgun, but you already have a dedicated shotgun. Grenade launcher would be my go-to. A zoom mode wouldn't really suit it, since it's not entirely meant for constant long range combat.
Blood's tommygun secondary consumes 2x the ammo per shot, but you wave it around, so you are able to hit enemies within a cone around the player's camera.
My idea for thrash was for a machine gun to have a hybrid between blood and the zoom in idea, where your fire rate doubles, you activate a laser sight, and you zoom in slightly. The downside would be to reduce player movement during that time. I don't know if it would really work out, but the idea was to be able to get a big dps spike at the cost of peripheral vision, long range precision, and movement
You could do like wolfenstein and allow the player to scope in and do powerful, accurate single shots at the cost of several bullets.
too many options.