When TOS movies started being made, they brought back all the Big Seven characters from the classic Enterprise. Khan and Sarek both had major roles as well. We also saw cameos from Chapel, Rand, Kyle and Amanda. Who else could they have brought back? I'm thinking specifically of the "Big Three" Klingons (Kor, Kang, and Koloth). Any of them could have easily taken on one of the big Klingon roles across several of the movies, especially since all three characters showed up again in DS9, all played by the original actors. I know Christopher Lloyd's Kruge gets criticized for being kind of hammy, but Koloth was a pretty hammy character as well, so why not put him in TSFS? Or in place of Korrd or Klaa in TFF? Kor could have been an awesome Klingon Ambassador in TVH. Or in TUC, why not General Kang instead of General Chang? I'm also thinking the Romulan Commander from "The Enterprise Incident" could easily have been brought back as a co-conspirator for TUC (instead of who-the-hell-was-Nanclus.) That actress was also still working, and was even considered for TNG's "Face of the Enemy" (she was unavailable during that episode's filming period.) Who else from the classic episodes could have made the jump up to the big screen?
Almost any character who survived their episode from TOS could have "made the jump", but I'm fine that they didn't. I'm personally not keen of this kind of thing because I'm not fond of "small universe syndrome", where this small group of people keep tripping over one another. Amanda and Sarek make logical sense (pun intended) as Spock's family, but the rest not so much. If I had to pick someone, it would have been Harry Mudd flying Valkris to meet Kruge, only so he could be blowed up real good.
I like the idea of McCoy hiring Mudd in TSFS since he probably doesn't know too many people who would take him to forbidden space. I wouldn't have minded seeing the TOS Klingons because Kirk had bumped into Koloth before -- so why not again? Also Kor or Kang may have wanted to face Kirk again and fought for the assignments that would take them into Fed space in hopes of doing so. And other members of the crew -- why not -- some sailors serve aboard the same vessel for years.
It would have been interesting to see Commodore Bob Wesley return... maybe Kirk would outrank him in the movies, if he never got promoted in the interim. That could be an interesting conversation or two.
Extra cameos on the Reliant, Excelsior, and Khitomer conference would have been cool. Leslie, Riley, Noel, Barrows, Kor, Kang, Koloth, the Romulan Commander, and it would have been nice if Chapel had been at Khitomer.
It would have been cool to see Samuel T. Cogley providing a counterpoint to the Klingon Ambassador's argument for the extradition of Admiral Kirk at the beginning of Star Trek IV, but then we'd miss the always awesome Mark Lenard as Sarek. Or, possibly as Kirk and McCoy's defense in Star Trek VI