I’ve been a huge Lost fan since pretty much the beginning, so the end of the show is pretty bittersweet.
I’m really glad that the firm end date allowed them to work towards the ending they wanted rather than just spin their wheels (e.g. the season 3 episode “Stranger in a Strange Land”), and I’m really glad that the show has stayed pretty consistently good. But, it’s still going to be sad to no longer have what I’ve considered the best network show on the air. It will be interesting to see if any of the knock-offs are able to hold a candle to it. So far, they’ve all been pretty awful.
As of writing this, I haven’t seen the finale yet, so PLEASE don’t spoil anything!

Sorry if this is semi-spoilery, but the finale kinda ruled out your idea; however there is one obvious spin-off, since the characters left on The Island at the end could do another “Fantasy Island” revival (hey, it couldn’t be any more disappointing than “After M*A*S*H” – NOTHING could)