Here's hoping that Desmond plays a part in going back to the island, too.
According to Ms. Hawking, the only great thing Desmond will ever do is trigger the fail-safe switch. While that doesn't mean he's exempt from going back (and let's face it, his departure from the show hasn't been announced) he was not one of the original plane survivors which means he stands less a chance of reinacting whatever is necessary to successfully return. Now, Frank is another issue, Frank was SUPPOSED to be on the island but something fucked things up down a chain of events. This led to the change in the pilot, Smokezilla eliminating an entity which was NOT supposed to be there, and Abbadon's insistence that Frank reach the island.
Oh and if you'd like to know what I think Abbadon might be, open this but I have to warn you, I think it's a decent guess so at your own risk.