David Brin
Greg Bear
Kevin Anderson
all have done sterling work in writing additonal novels in other authors universes recently, Bear and Brin did some good prequells to Asimov's Foundation series, and Anderson does fantastic Star Wars books and has co-written the recent prequells to Herbert's Dune series.
others I would like to see, Ben Bova, Neil Gaiman, Terry Pratchett (he would do a great Banzai Batallion, and anything to get him away from the dire discworld stuff)
oh oh Jeff Noon - check out his vurt, pollen and autonmated alice books
or how about Ian Rankin for a good crime story (brit cit judges)
Peter Carey for a chopper in OZ story !!
Phillip Jose Farmer for a true helter skelter story
and if you could bring a couple back from the dead, Edgar Rice Burroughs for a slaine novel
oh oh Marion Zimmer Bradley for a slaine novel
William Burroughs for a real dope story (sorry Gordon)
Christoper Tolkein for any good fantasy stories (what he's done with his dad's work is fantastic)
oh I can keep this up for ages