As the title says really. 

Steroids or something similar to aid in gym gains. These can come with an addiction or a an OD (with excessive use) - our own docs can cure or perhaps we can produce and sell something to bring the levels down. Studying the medical tree could bring bonuses to addiction.  It could benefit newer players too - the less stats you have the more you gain ininitially. Plus it's something else to use the cash for.

I'm just thinking out loud.

I think this is a wonderful idea.

And it could actually get real interesting to more complicated it got down the road.

For example, if you take too much steroids in a certain amount of time and also within a formula that took into account your intelligence, then the steroids could actually lower the amount gains you get.

You could call it an abuse system, where if you take too much of something that enhances like a drug then you can actually go downhill and it affects how much you gain

And rather of different gyms, you can have one gym but many different gym types of equipment.