By turrets do you mean something like
CROWS mounted on a little bunker or CSWs like the Mk19 and M2HB? CROWS might be too unbalanced since heavy weapons like that should be high risk high reward. Weapons mounted on tripods could serve that purpose easily.
Concrete pillboxes, if small enough, could be good to have. Preferably with limited fields of fire so that it's actually possible to take them out. Otherwise entrenched foxholes should do nicely, since you're more open to fire and grenades but you have a better field of fire.
The idea of 'side missions' appeals to me, perhaps semi-randomized like a helicopter crash landing and you have to rescue the pilots and crew before the enemy team finds it. Or freeing POWs or VIPs from a randomly generated encampment thing. Completing side missions could grant you mortar fire missions or other goodies like enabling an APC spawn.
As for weapon attachments... nah, there's plenty of different kinds of guns that can fulfill different roles while being distinct from each other in various ways. Having a G36 and an M4A1 with a grenade launcher, for example, is much more interesting than having an M4A1 and an M4A1 with a grenade launcher.
(Pssssst. Pasik... add in battle rifles. I want me an M14 of some description >=3 )