I believe that Adam and Eve are Jack and Kate.
That is where they will end, and where they will begin. Sure right now, it will look pretty lame, Because EVERYBODY is expecting it; but when we look at the show as a whole, and not just a season by season story, we'll appreciate what the writer's did, and realize just how awesome that would really be.
Rose and Bernard.... Really? These character are fun to watch, but the mythology of Lost? uhh... No.
Unless they are a huge plot line in Season Six.
P.S. Jack and Kate's relationship in the real world sucked (4.10) their life belongs on the island, its where they want to be!