I don't think a level playing field is even necessarily desirable. Picture two perfectly matched opponents cancelling every maneuver and strength, essentially engaging in a stalemate. Complete parity would make fights boring.
There are a lot of variables that contribute to a fighter's advantage. There is talent, physical build, coaching, teammates, experience in the cage, length of time competing, mindset, work ethic, financial backing, and medical attention to name a few.
However, there are two topics that just have to be addressed: Steroids and weight cutting.
Recently there have been a couple failed drug tests, and a big time failure at a weight cut. All three incidents raised the question of what needs to be done.
Purportedly, there are a number of fighters already on TRT, which segues right into what seems like an obvious solution. Establish a baseline level of testosterone that is equal to the highest naturally occurring level accepted by the medical community, and let everyone supplement to that standard. Of course this would require medical supervision, and the UFC should be the one to provide for such monitoring.
Now for the weight cutting. I think there should be a standard weight for each weight class that would have to be met and maintained two weeks out from an event so that a fighter could only cut a fixed percentage of weight from there. The percentage would be based on a mild sliding scale to compensate for increasing weight classes.
I think both of these ideas would serve to protect the fighters' health, and provide for better competition and performance.