Party Bear i think the infantry (and by infantry i mean mechs) is acctually quite good for their cost, at 3000 a pop you can make 2 mechs that would cost 1000 less than a tank, and would win 2v1 against that tank every time, even if the tank struck one of the units first. On top of that, mechs can get the mountain defense bonus, which is insane for them. They are also valid vs AA guns, 2 mechs is 2K cheaper than an AA, though they don't give you any air defense. Mechs are meant for base capture, rugged terrain movement (nothing impedes those 2 movement points they get, although 2 movement is pathetic, BUT its only pathetic for reinfocing an area, in the thick of combat in an urban/forest/ or mountainous area its enough) and light vehicular/ small tank/ and infantry combat. They can also be used for squating on a barracks/airpot/naval yard or suprise capture, but normal infantry are more effective for that.
What i think there needs to be is more infantry classes. maybe weak AA infantry, or a spy/sabatoge unit, maybe a demolition unit.
And about the tank killer, if they do make one, then what becomes of the ranged units? I think they'll lose a lot of effectiveness. Big tanks should only be fought with bigger tanks, or air, or range, at least from a balancing perspective. However, i didn't like the neo or mega tank editions, i think heavy tank and regular was all that was needed from the tank class.