If they knew that the show was going to end I always thought a great final episode would have been to have them find out they were on one of the Hawaiian islands all along. lost in their own backyard yard so to speak.

If Gilligan's Island hadn't been cancelled, the castaways would have turned the island into a resort hotel (https://www.metv.com/stories/if-gilligans-island-hadnt-been-cancelled-the-castaways-probably-would-have-turned-the-island-into-a-resort-hotel)

It's a story of rags to riches.