If it was past authors: Lovecraft and Frank Herbert
Oh, if we're doing "late" writers, I'd love to see what H. Beam Piper, A. Bertram Chandler, Poul Anderson or James H. Schmitz might've done with a Star Trek novel. Anderson wrote a Conan novel in the late '70's, so it might've been possible.
I'm pretty sure Marion Zimmer Bradley wrote some Trek
fanfic back in the day. I'd LOVE to read that. (I've always thought Ann Crispin's Yesterday
duology read like "Star Trek
Back to the living: Allen Steele would've done interesting things with Belle Terre, had he written one of the New Earth