It's not invincible, it's meant to get get rid of other infantries easily. Since it has low health and no range, it will get destroyed by roman and marines, making it immune to those attacks so it could get rid of marines and romans efficiently. A chronite tank would nearly one hit kill it, but it regenerates quite fast. So, against 1 tank, it could win with difficulty, against 2 it will die before it reaches the tank. It's quite fragile really, far from invincible.
-paratroopers: could drop behind enemy lines and be effective against snipers / medics
-devastator: slow expensive giant brute tank with big guns against ground and rockets against air and killing nearby (also friendly) troops when destroyed with its explosion.
ok but really, the base should be given a third weapon, a laser that "locks on" to a targit and slowly powers up to be very strong. at first it would be weak, but soon it will far out damage a mod-spiders healing (the reson why i sujested it, it will stop power unit rushes, but be more or less useless in the later game)
some difrent marines or more marine abilits would be nice, i wouldent mind seeing a marine fire of a morter/grenade every once and a while. think a slightly weaker form of there anti air rockets aimed at the ground i guess.
to add another start-craft idea, some from of unit that has to unpack to fire (sige tank? lolol. or if your an AoE fan, tebuchet? lolol) made to defend. hmm, as long as it would still easy eough to kill so that monarcs dont have an inf hog.
I would like to see a super healing unit. Like a super meditec in a kind of way. I would also like to see a kind of game type that only allows infantry.