I'm a nut for more and more units so something after machine gun would get my vote. I already posted some ideas for conventional automatic weapons (I love the idea of an autogrenadelauncher), but I wouldn't be against RPGs, SAMS, and other hand held/vehicle mounted missile weapons. However, I think the weapon type inherently creates a problem for unit funtionality. RPGs = anti air, vehicle. Grenade launcher = anti infantry, vehicle. Which one is the superior choice?
How about a mixed unit with a generic name like 'Support'? It would incorporate missile and large calibre weapons that can damage the different units classes equally. This is assuming that a defensive line unit like the machine gun is wanted of course. What do you think?
Eh look at my suggestion; the Technical as the upgrade to the Machine Gun. It becomes a hit and run unit, to harass your foes, a modern day horse archer with good movement and ranged attack, but weak strength, vunrable to tanks, helecopters and other fast moving units, yet but cheap and resourceless, meaning they could be spammed in an emergency. The technical can be outfitted with anti armour, anti infantary and anti air gear, so type is not of a matter. But is it right to upgrade a two move per turn unit to a 5-6 move per turn unit (those Toyota can really rocket)?