SMG's anyone? I only think this would be a cool addition because well of the time period and the type of combat that we're facing. Maybe some kind of 'mid game' research option that allows for smaller, more compact machine guns that take up less weight but produce around the same firepower.
Secondly, what about some kind of close air support craft? I know the Huey was still in service from vietnam and the F -14 I believe was relegated to this role as well. I just think that it would be more interesting to have more involved in the 'over world' sense. Like if in the 'air' part of the combat the main craft's escorts weren't dealt with properly then they could help out when you tried to assault the downed craft... The aliens are interested in strafing highways after all.