A couple of things would make Infantry far more museful. More cover near bases would be great. This way, and early Marine build would be a great way to turtle up for a short time so that better units could be made. Along with that, the Cougar (an APC) should have been included in the game. One of the main problems with Infantry is thier speed, so introducing fast moving APC's to transport them would have been awesome.

Also, the Y Ability never should have been an ability to begin with. Marines and Grunts should both have an RPG (Fuel Rod) upgrade, which would increase their effectiveness against air by a lot. Rather than the Active RPG ability, it would just be a Passive ability that increases their damage by heaps.