I think the 3rd film would benefit from some actual villian casting too. Pick an actor with some charisma.

Paul Rudd would be a great Ghostbuster.

A new villian would be best. Why rehash the past.

This new start reminds me of Marvels Ultimate series... which over time became a slave of following the path of the original universe with a convoluted history all it's own. Hopefully Star Trek doesn't follow a similar path.

A new alien race would be great.

Khan is boring anyway. Oh boy, he's marooned. Big deal.