There's a lot of banned substances in MMA, mostly based around performance enhancing drugs. We don't really want situations where one fighter has an immense advantage over the other, because they were taking anabolic steroids. We also don't want fighters becoming drug addicts, and harming their health. So we have the WADA or World Anti Doping Agency.
What if we didn't have the WADA? Hear me out. What if we got to see fighters compete on any type of drug. Let some fighters take as much steroids as they want for a year before the fight, give them a bunch of PCP and Meth between rounds to see - how hard can humans REALLY punch!?
There could be a fight card specifically for different kind of drugs. One bout would be like Nate Diaz vs some other stoner fighter, where they have a smoking competition before their MMA fight. Then we could get say two fighters to have a drinking contest before their fight, just watch two dudes smashed off their face missing hella punches.
This will probably never happen, and shouldn't ever happen. Except the smoke contest stoner fight, that one should be allowed and wouldn't hurt anybody. Hell, you won't even notice the difference with Diaz.