I think that we need an upgrade to the basic standard sentry to be a machinegun that just throws many bullets down range full auto to defend the FOB - with standard sentry range (a full screen preferred).
I want that feeling of 'oh my god the sentry ammo indicator is going down' and the BRRRRRRRRRRRRRr of the sentry and the intimidation factor for benos. Balanced by the fact that it's going to go out of ammo and need regular ammo replacements and babying from the engineer
BRRRRRRRRR