What if Aaron is Jacob? I know.. i hate those stupid kind of prediction like Boone is the island and MIB is Nikki just there to piss everyone off. But stick with me.
Going on the basis that the MIB can shape shift into the dead on island, then he was using Christians' form.
Now Christian lead Claire away from Aaron season 4 and told Locke that Aaron is where he was "supposed" to be. Which lead to him being off-island.
Then Claire appeared to Kate in her dream and told her not to bring Aaron back to the island. If this didnt happen, Kate would certainly bring Aaron back to the island on 316. What if Claire was MIB? Is it likey that Claire died? Considering she was so chill at the cabin im guessing she wasnt quite "herself". You might ask how can MIB manifest as christian and claire at the same time. Good question. Smokey (who is the MIB or his minon, i believe) is made up of three little smokeys. This is shown when Juliet and Kate confront Smokey at the fence. If you watch that scene smokey splits after hitting the fence. They actually have a horse/dog shaped head which looked cool too. ANYWAY. Thats how smokey/mib can occupy both christian & claire.
I believe John was never meant to be brought back, if we look at Jacobs best interest. (More on this later)
SO without Aaron and John instead, on the "return to the island" flight Aaron would miss the the FINAL flash. what flash? the flash caused by Jughead. The flash that will cause both groups, 77's and 08's losties n co, to travel further into the past.
IF Aaron WAS on 316 and not John Locke, THEN Jacob would live and both Aaron and Jacob would travel to the past. A past where Aaron grows up to be Jacob himself. THIS MY FRIENDS, IS THE LOOP! Keep listening...
Because there is no john locke for MIB to occupy, MIB cant not get anyone to kill Jacob hence. Jacob would live in '08. and '08 Jacob and Aaron flash to the past where Jacob can teach Aaron as he get older everything he knows about the the island and the future and what to do to keep the loop in place.
BUT now that the MIB has prevented Aaron from returning to the island and has killed Jacob, the loop is broken. AND ALL HELL IS GONNA BREAK LOOSE!
Thanks for reading =)