I would suggest tiered/gated upgrades...
-minimum # of particular units @ a particular level to upgrade to next tier
OR
-specialization upgrades - gated to # of units required and at level OR additional support units to make this upgrade possible
New units - again tiered/gated
-fighters (air support)
-multiple missle launchers (splash damage)
-Vulcan upgrade to RAILGUN (should be expensive, but will definitely be a hard hitter!)
great game, love the sound and graphics...
NOTE: I'd be willing to purchase additional upgrades in this manner (don't mind supporting your efforts)
