I'm thinking it would've been nice to have had several other new additions to the crew (the E-A's "next generation"), who would be the ones taking over from Kirk and Co, after they retire, but are onboard for one mission with the living legends. This could've widened the pool of suspects and not made it so obvious that the one new named character was the traitor onboard. I know this may have stalled the pacing, as the newbies have to be introduced and fleshed out enough for us to give much of a damn about them, but it could've made the situation on the ship tenser with the old guard and newbies wary of one another, no one quite sure just who is involved. Early developments of TWOK included several new officers on the crew, so some of them could've been included in the mix: Charles Waters (weapons), Diana O'Rourke (comms, of course she is attracted to Kirk), Thal Arctors (navigation, a human/polar bear somehow, who enjoyed whispering Elizabethan poetry into women's ears), and Uti Sorbayo (medical, East Indian, religious, and interested in McCoy). Any combination of them, along with Saavik and/or Valeris and you'd have an interesting mix of potential suspects.