That is what Classic is, except they are using 75% as their consensus threshold (it is arbitrary though - if you maintain 51% of hash power you can dictate the protocol).
It's not really 51%. The miners are supposed to accept the "tallest" chain. So you can get lucky and mine a block then change the rules and sneak in some breaking change (like a larger block). No one else will accept it unless you can keep mining quicker than everyone else and eventually convince everybody that yours is the "true" blockchain so everyone else should switch. The thing is if you have less than 50% of hash power, it's extremely unlikely you'll be successful because 50%+ other miners disagree. Once you get above 50%, you're chances go up but you're still playing with fire here because just a short period of bad luck can end up breaking your "fork". Even at 75% there would be some big issues with the 25% left behind who might not initially realize what's happening (and maybe get lucky).
The fork accepting more kinds of block is probably going to be jittery and broken right at the switchover no matter what. The percentage only affects how fast things stabilize. 60% would be fine after a couple hours. Requiring 75% or more is to smooth things on a political level.
