Bringing back the original cast is often seen as a cheap ploy to save a flagging franchise, and though the franchise is most certainly flagging, it wouldn't feel quite so cheap if the script is in the right place. I have no idea what excuse could be thought up for Dr. Grant, Dr. Malcolm and Dr. Sattler to return to the island, but damnit, I'd really love to see that. I think it's fair to say that Richard Attenborough's frail condition would mean that he couldn't appear in Jurassic Park 4, though it would be nice if there a reference to him, perhaps writing his character out by dying of old age. Another idea is the return of Tim and Lex as adults, though again, bringing them back to the island after their initial childhood trauma could be a bit of a stretch. Bringing the doctors back is the most likely possibility given their professional hunger for the dinosaurs; perhaps they can visit the island with an armed detail this time, who nevertheless still find themselves in over their heads.