It would be very cool if you could add actual tank guns. Turrets and chassis are available to everyone right now, since they can be built using fuselage blocks, rotors, wheels and so on. We're only missing tank tracks and wheels, but that' another story. They would have a few customizable parameters:
1. Caliber (in mm or inches)
2. Length (in calibers, like L/55 which means 55 times the caliber)
3. Smoothbore (worse accuracy, higher dmg) or rifled (higher pen, better accuracy, lower dmg)
4. Standard shell: APHE, APCR, HE, HEAT, APDS or HESH
5. Muzzle brake or fume extractor
6. Autoloader (loads automatically), drum-fed (single-shot cannon that fires from a drum), autocannon (fires a salvo of rounds automatically) or single-shot.

Regarding point 6, I recommend you check out the following tanks for clearing it up: T-14 Armata, AMX 13 90 or AMX 50 100, M5 Stuart and Tiger I (or Tiger II, aka "Königstiger")