ok, so this is all I have, it isn't much, but its something. I think I'm missing something though, I seem to remember another article where Jim did discuss a possible sequel, but I think that particular one is on-line somewhere. Who knows, but after some digging, this is the only inkling of a sequel being muttered by Jim:
"I don't see a Dark Crystal II" said Henson, to tumultuous applause from an admiring audience at the World Science Fiction Convention. But the puppet master clearly left open the possibility that a follow-up film might feature another story set in the same fantasy land, perhaps a better story this time now that hindsight shows that characters and setting alone aren't enough to support such a fantasy vehicle.
Also one other part:
All the props, costumes and scenery from The Dark Crystal are carefully stored away in a massive warehouse in Borehamwood, England, nearby the EMI Elstree studios. Henson didn't have the heart to destroy them or throw them out. "I just couldn't do that because I love them so," said Henson. "Everything is there for us to start up again."
So, that's it. This comes from "Cinefantastique" Volume 13 Number 4 April-May 1983.
So, it looks like at one point, he kept the option open, but it seems the story he would have liked to do didn't hinge on the crystal, or Gelflings or another round of Skesis. But a whole new group of characters going on their own adventures. Which makes much more sense then to 'crack" the crystal and have the whole mess start all over again. It borders on a re-make or rehash of what was, already, such an original story.
But hey..who knows now. So, there ya go.