Sideways world shenanigans... This is important. Here is what I think needs to happen. They need to unlock Locke. They need Locke to be ready for the sideways world. This is why Desmond didn't just try to awaken him once but twice. I've thought Locke did know but is denying it because his life sucks so much in the original timeline. I don't think so any longer. I think it has to do with MIB. What if MIB is suppressing his memory and mind? There is something that sideways Desmond knows about Locke's importance in the other timeline, and there must be a way to get him to slide over and mess with MIB. What if there is no killing MIB, but if Desmond can get sideways Locke to realize the original timeline and slide over, could Locke's existence in the original timeline mess with MIB being in his form. Something interesting was how Desmond's consciousness switched timelines and could switch backward in time (Brutha) so that his mind was what was jumping timelines. Well John Locke's body is dead in the original timeline, but MIB uses his form. What happens if Locke jumps timelines and occupies his living form again? What if that can kill MIB since two souls cannot occupy the same body at one time? The 'concert' that the main characters are going to will have all the main folks, Farraday, Farraday's mom, who always knows shit, Widmore, and something will happen there. I think it is a mass awakening of the castaways and Locke's slide over. I think this will let Locke jump over and destroy the MIB. MIB has a soul but no form. Locke has a soul and would take over his own form, it would be an inward battle but the only way they could 'kill' a frickin smoke monster.
Original timeline... It really is wide open. I think Miles got Desmond out of the well. Jack, Sawyer, Hurley and Kate catch up with them. Ben and MIB hunt them down. MIB can't kill the candidates, but now that Jack is the new protector of the island, his focus is on Jack. This is going to leave Ben to kill them. He has a choice. He's also one of the best complicated characters in the history of tv dramas. I think Ben dies a horrible death at the hands of one of the Losties. Not before Kate and Sawyer bring up 12 yr old Ben being shot by Sayid and saved because Kate/Sawyer made it happen. MIB will still be there offering him the island, even though he told Ben he'd use Desmond to destroy it (which he won't have to if he can kill the candidates). MIB will tempt him, and original Ben has always been about power for himself. The "you're mine" rant with Juliet, stealing Alex as a baby, whatever he did to get Widmore off the island, and his treatment of Locke (shooting him, hello), and killing Jacob all point to a guy who tries to be good but is always selfish. LOST does not reward the selfish. This won't sway him and Hurley will have to eliminate him. Yes, Hurley. His character development has lead to him being more proactive, assertive and aggressive. It makes sense that he kills the guy who manipulated Michael, which killed Libby, and then threatened his friends in the finale. Yes, I think Miles says a message from Ben after Hurley kills him, like "bury me next to Alex".
This leaves MIB to deal with them himself. He then goes to plan B, use Desmond to destroy the island. MIB knows where the light is, but i think he's going to the wheel. Desmond is gonna give some "Jack, brutha, help me". Jack doesn't know about the wheel, but Richard does. Richard will explain why MIB wants to use the wheel and say what the island is. Richard will take jack to the wheel room station. He will get there as MIB has Desmond in his grip. MIB will release Desmond. Jack will run to them and yell as usual. Desmond will say to Locke "have you seen the light brutha", and Locke, now back to being Locke and not MIB will say "yes, I understand". This brings sideways Locke to our timeline and destroys the MIB. Resolving the storyline of the island.
We get a 22 min section of character wrap up. Locke will stay on the island with Jack because Locke has never wanted to leave. Locke will lead the others and whomever wants to stay on the island (Rose & Bernard are two). Sawyer, Kate, Claire, Miles, Desmond, the dog Vincent and some extras will leave the island on the plane. I have no clue how they will get off the island, but it will happen. Claire and Kate get back with Aaron. Sawyer meets his daughter. Desmond will be reunited with Penny, and they will tell Eloise Hawking that all is set right. The finale ends with Jack and Locke on the beach by the stone statue just talking, smiling and laughing.