This is probably impossible to implement, but:
There should be a biter tech tree, which uses bottles of pollution in special labs
There should be a biter specific resource that is needed for players, and a player specific resource that is needed for biters, so they would both have a reason to fight each other.
I like the thought of one building used for everything for the biters, like you assign them uses, say one creates biters using pollution and iron you got from the player, and then another building harvesting pollution to research
There should be a goal for the biters like the rocket for the players. A strong weapon will be locked behind whatever you get in the launch (say you get nukes only after you launch a rocket)
Add a lots of weaponry, and make the weaponry the biters make not usable by player vice versa.
Examples for the ideas: 1. Every person building makes a type of pollution, and then you can harvest the pollution for s...