[Spoiler-free, theory based on episodes up to The Package]
It would seem that many plans are in action at the moment. MIB wants to fly the plane off the island with the candidates; Richard wants to destroy the plane, containing the wine in the bottle; Jack wants to get people off the island, promising to Sun to get her and Jin onto that plane; Widmore, the only person who is actually within reach of the plane, has a plan that somehow involves Desmond, but wants to help contain MIB; and Sawyer rebelliously plans to take the sub.
If the submarine becomes missing in action (which I think is likely, why wouldn't MIB have planned to use it otherwise?), the only way off the island is the plane. However, the plane's pilot is Frank, holding true to Team Jacob and seems to be steadfast. That's a lot of pressure on a not-so-main character, in a tangle of storylines. If anything happened to Frank, what would MIB do, and why hasn't he tried to recruit him before?
The point is, everything is revolving around the plane. It's the key to keeping/spreading the darkness of Season 6. And how do you even destroy a plane? Hopefully somehow it's epic. I am making a huge prediction on what is going to happen.
An important scene from Pilot - Part 1:
JACK: We must have been at about 40,000 feet when it happened. Hit an air pocket. Dropped, maybe, 200 feet. The turbulence was … I blacked out.
KATE: I didn't. I saw the whole thing. I knew that the tail was gone, but I couldn't bring myself to look back. And then the, the front of the plane broke off.
JACK: Well, it's not here on the beach. Neither is the tail. We need to figure out which way we came in.
JACK: Because there's a chance we could find the cockpit. If it's intact, we might be able to find a transceiver. Send out a signal, help the rescue party find us.
KATE: How do you know all that?
JACK: Took a couple flying lessons. Wasn't for me.
KATE: I saw some smoke. Just through the valley. If you're thinking about going for the cockpit, I'm going with you.
JACK: I don't know your name.
KATE: I'm Kate.
That's right, that's my call. Jack, the unknown second pilot, the de facto leader recently converted to Jacobism, flies the plane. Whether he flies all the candidates off the island without MIB, or turns to the darkside and flies MIB 'home', or he flies the plane into the water to stop MIB in a leap of faith, or just finishes the flight to Guam, is anybody's guess.
I understand there are theories that encompass the entire show, or just solve one or two mysteries. I'm just trying to predict what's going to happen in the last hours of the show. Comment at will.