Suggestions? Mine would be an engine size limit on turbo cars.
However I think the Mcalaren Chev dominance may have played a part in the implosion, Ford and others never really got any kudos that might have encouraged greater efforts. If Ford stock-block engines, perhaps a 351 with Gurney-Weslake heads like the Honker had, had a win in 1966-67 then perhaps Ford might throw a touch more effort into Can-Am and keep the series a bit more interesting for a bit longer.
There are three problems with this:
1) Limiting the turbo engines won't stop Porsche from bringing out the 917/10, they'll just develop the flat-16 engine they designed for it instead if they can't get the horsepower they want out of the turbo engines;
2) The Honker was a piece of shit of a car (Mario Andretti referred to it as the worst car he ever raced because it was horribly off the pace and its handling was absolutely diabolical) that never had any business racing in the first place, which is why after it Ford went with McLaren chassis with their engines in the back. There really isn't a way of redeeming the Honker or the F3L;
3) It doesn't do anything to deal with the explosion in costs that the Can-Am series suffered in its later years. The Shadows and the Chaparral 2J were technically brilliant but like the 917/30 cost bonkers amounts of money to race, and when the energy crisis hits those cars will simply not be viable to race because of their costs.