Thing should have differnt classes.
For example only the thing/abberision can costume to gain hp
Zergling thing is a super fast thing and can attack quickly and run away, like a hit and run class
Brutalisk/ultralisk can be tank class who walks slow and it has to utilize burrow charges and maybe come with a charge ability, it can tank more hits because it has more hp and armor, it can destroy doors much quicker
Queen, a spell caster thing who can create like 4 weak Zergling that can scout and attack, is ranged, can throw like smoke bombs/produce black fog like lights out to disappear (it blinds u for a time)
Changeling, weak fragile thing, it's marine form is its thing form, it has two attacks one that makes spikes appear and other is regular marine shoot, can be cloaked, can change to any marine color even alive
Infestor, can attack marines and leech onto a target in a range and can use his life (if it's hp goes down it can use the marines hp and the marine is injured in process) can use fungal to freeze and deal damage to marine to escape.