zerg is the strongest race by far in sc2 for a few reasons
-gas free production that is part of eco and defense (after tech is built)
-very strong units that win most a-move scenarios
-unbeatable map control thanks to extreme mobility and massive map vision (except for timing attacks and turtling expanding)
-strongest spellcasters with out of fight (easy) micro, ie abduct and parasitic bomb being used to snipe when nothing is going on
to balance zerg doesnt have to be touched. some general changes can be implemented.
-zero gas cost production structures/add ons/tech structures after the first one is built (all 3 races)
-recall on nexus cooldown not global
-terran gets hercules with tactical jump (targetable to own ccs only)
-vision abilities buffed for terran and protoss, ex: zealot or probe hallucinations last forever, ravens can deploy sensor probes.