Besides the obvious cc for cc racing irrespective of engine type. Could be a rotary diesel for all I care.
Mine would be this: Proper Production rule. By this I mean factory teams must run almost everything OEM style. The only exclusions to me would be new spring rates, valving (suspension only) and tires. No more several-hundred-thousand dollar bikes. The bikes MUST weigh within 5 pounds of what it does coming off the showroom floor. Everything engine wise including exhaust must be 100% original factory. No engine work, nothing besides spring rates valving and tires to suit the occasion.
This would serve a couple of purposes if you ask me. For starters, get rid of these rebuild every race 250Fs. Its bloody stupid. It sends millionaire teams BANKRUPT FFS!!!!! I mean when a sport that is supposed to be open to most people is sending millionaires bankrupt, something needs to be done. Secondly, it gives the privateers a more even playing field. They can run their engines for a good 40 hours before rebuilding. Thirdly, it encourages manufacturers to FIX THE ISSUES OF A BIKE AT THE FACTORY!!!! Eg: 2009-12 CRF450R. Completely Useless in almost every department. But it didn't hold chad reed back did it? He was running alright on it. Cos he had a couple million dollars to spare. Fourthly it brings more rider skill into the equation. If a rider has to run what the factory blokes brung, flaws and all, its bound to throw in some interesting line choices and it shows viewers that just because Monster energy/Pro circuit Kawasaki team has won the past god knows how many championships it doesn't mean its the best bike. Although according to MXA it is lol.
Now maybe I'm just being shortsighted. I know that alot of the appeal to watch pro riders for some people is in these crazy tricked out bikes they ride, but I don' think thats what it should be about. Maybe if this rule was put into practise, all the factory teams would tell the AMA to eat a d**k and pack up and leave (Assuming they didn't just buy the rule out) and the market would crash. Its hard to know.
I think it would help in so many ways, but of course theres downsides to anything.