I really do not want to see the Klingons as the focus in the next film. As background characters, fine. But the Klingons are the Trek race that has been most done to death.
That might actually be a reason to use them: they are familiar to even casual Trek viewers.
If these movies are going to focus around Kirk & Co. then the logical choice are
the Klingons. They were the major foil to the original crew, especially Kirk. I would hope to see Kor or Kang, or Koloth & the coming war with the Federation, then maybe an early attempt at peace. Spock Prime could lead that effort.
The other idea would be Q only because some have speculated Trelane was a member of the Continuum.