Looks awsome! I got an idea!!
How about, if the aliens become stronger, S2 vs S1, the droid upgrades itself to S2? or you just rebuy it? Droid, S2 Droid, S3 Droid...
If aliens are S2, and humans S1, the droid gets upgraded the next time you use armory... Meaning, droid becomes faster, fires much stronger, and turns fast, and yeah...
If aliens are S3, and humans are S1, to balance it, the droid upgrades to S3... So it will do a significant amount of damage!
If humans are the ones to become stronger, then the droid weakens... you get my point? This is a truely balancing modification that would make things even!
Also it will cost less and less, each time aliens get stronger, and humans stay the same stage. So it would be...
S1 H vs S1 A = 350$
S1 H vs S2 A = 300$
S1 H vs S2 A = 150-200$