The big problems with the films are they are both boring revenge stories and the time travel element is essential to both plot stands you have to unpick and reweave.

The action sequences are also of a similar character so if you want a 'best of' selection you are stuck with a choice of Jam or Jam.

Cumberbatch playing Khan is dumb if they had made him another member of Khan's crew fighting to secure the safe awakening of his leader I could buy that but Khan is an ethnic Sikh and his character has already been done to death.

Nero, is more of a clone of Khan/Shinzon (also a clone of Khan more than Picard) so it would make more sense to just drop his character but then you need to change the big ship to The Vengeance which would require new special effects work.

I fear an ideal combination isn't possible.

The only fusion I can see working is John Harrison being the person flying the ship that destroys Romulus and sends himself and Nero back in time.

It might be possible to have him land on Nero's ship and have that consumed by the red matter.

That way the timeline is changed but there is only one Spock.

Khan isn't in it. Kirk doesn't die/get magicked back to life. Scotty is on the ship in the beginning, no transwarp teleporting,

You get the best of both villains to work with and you can edit some of the Vengeance crash to be the effects of Shinzon (sorry Nero) firing on earth.