I think another interesting question is, which TNG episodes absolutely would not have worked as TOS episodes?
Obviously all the holodeck stories are out. I couldn't see something like "Best of Both Worlds" working, either.
It would have worked, but it would have been incredibly silly in the 60s. An alien machine-race (humans in gray make up making robotic moves and speaking very strange) attacks the Federation. Kirk gets turned into one of them (he's wearing gray make up, making funny robotic moves and speaking very strange). And in the end Spock performs a mind meld to get in contact with Kirk and puts everyone of the aliens to sleep.