An armored vehical system. Make 3 basic chasses and put corresponding turrets on them. Would be easy and lots could be made.
EX:
Light chassis: Light aa flack, light tank, tank destroyer, apc, scout, Ambulance(heals on entrance)
medium chassis: IFV, medium tank, Heavy aa, Artillery, assault gun
heavy chassis: Heavy tank, siege artillery, SAM carrier, mobile missile launcher.
You could make a large amount of vehicles with minimal modeling needed