BC's character should have stayed as John Harrison. The guy is flawless, now that I have seen STiD multiple times, I strongly believe that he should get nominated for an Oscar, If they could nominate Ledger why not him?
The only thing that spoils BC in this film was that he was Khan. Am sure with some of the backlash STiD has received, Orci and CO are probably regretting it and pulling their hairs out about some of the bad decisions they made in this film. (Another bad decision was the Khan scream by Spock) However this is just my personal opinion.
If BC was just a brand new character to the Trek Universe, it would have been beyond genius. BC was just so great in this film. All Trek fans were blessed to have him. Apparently some Star Wars fans are begging JJ to cast him in episode 7.
If only he had not been Khan. Trek could have created a brand new iconic villain for this generation solely because of BC's great acting, cool demeanour and enduring presence on screen.