Yeah I have thought a lot about different ways of trying to simulate the mutant system.
One way was to have it that only captured slaves can become Mutants, but I am not sure how good the AI is at capturing slaves.
The other problem is trying to make Mutants 'unbuildable', but still cost money to upgrade..
The way to make them unbuildable is to set their cost to -1, but then that also makes them free to upgrade.. (or perhaps unupgradable as well... I am not sure if I ever worked that out..)
We can make an improvement that spawns a mutant on build or repeatedly over time.
Ideally I would want all mutants to be unbuildable and you just pay to upgrade their weapons and equipment, I am just not sure if that can be done because of the above problem..
Another alternative might be to give mutants lots of commerce and little hammers, so that Mutants are 'almost' impossible to build. That does not seem like a very good solution because evrything would be near impossible to build...