If you have any ideas that you haven't had the time to think through or just want to give down-right-silly suggestions, feel free to post them here.
I'll get the ball rolling. I suggest being able to evolve into an Overmind. Upon doing so, the previous Overmind promptly explodes. It would have no extra features other than the ability to view others happily move around while you're pathetically fixed to your spot.
For the sake of balancing, the humans are able to buy a Reactor-suit. This CAN move.. but only just.
Unlike the Overmind, the Reactor-suit is more like a repeater as it does not generate it's own power.
Like the Reactor, the Reactor-suit is fitted with a 'zap', and no other weapon can be held while wearing it. Nor can you buy armour (as the Reactor-suit IS your armour). You can't buy a battery-pack (because you are one) and you can't buy a grenade (because I say so). You can however buy a jetpack. This, like walking, is movement.. just very slow movement.
Also, dretches explode like nukes and humans are made of chocolate.
Who knows, one of your half-baked ideas might have something suprisingly inspiring in it.