BRING BACK THE NECESSARY CHARACTERS
While it would be great to see everyone back, but if the film is going to be 2 hours plus, then you can only do so much. If characters are dead (except for The Lone Gunmen who I never believed them to be) keep them dead. Bring them back only in flashbacks, something to help convey the story where necessary.
Doggett, Reyes, Skinner, Gibson Praise, Marita Covarrubias, Strughold, Billy Miles (maybe leader of the alien colonists?) and the Alien Bounty Hunter are some of my suggestions that are needed for the next film.