Nothing against Walter, but I wish they'd never inwented Mr. Chekov at all. Star Trek movies would never have suffered Chekov's campy accent, to say nothing of the obvious difficulty that WK had in delivering a genuine, believable performance while doing the accent. It must have been like flying a plane and taking a dog's temperature at the same time for him, because neither thing came out well.
With no Chekov, Walter would have surely found other work in roles he was better suited to, never gotten type-cast, and maybe even become a star.
But rather than make Riley a regular, which I would have supported at the time, I now think ST needed another woman in the cast, provided she could have been written with our modern take on the fully-actualized, competent modern female character who gets a fair shake. You don't see that in TOS except for a few aliens like the Romulan commander, and we only see her on the day she makes the biggest fool of herself in the history of the Empire, mooning over an exotic man.
Dr. Dehner in "Where No Man" and Dr. Noel in "Dagger" are the show's best models for what a strong woman in Kirk's crew could have been like. Give us that instead of Chekov and the cast would be better balanced.