AHHH IT'S ALL KINDSS OF EPIC

Mabel's bubble is a trap for Dips,Wendy and Soos. I mean, if Bill really wanted to keep them away from Mabel would he place her cage in the middle of the town where everyone can see it and have a 10 year old guard the key?
There's also been some theory's floating around that Mabel is powering the apocalypse
ALSO THE CRYPTOGRAM FOR LAST EPISODES READS "WHICH PINES WILL SURVIVE" NO NO NO YOU ARE NOT KILLING MY PRECIOUS CHILDREN

There's also been a theory floating around that I LOVE stating that Bill put her in the bubble and gave Gideon the key because A) I'm fairly sure Bill believed that Gideon was so obsessed with having Mabel for himself that he never dreamed that Gideon would give up the key in any way B) Some people think that Bill has her locked up in this giant bubble with chains and a lock on it because he's SCARED of her. If you look at her bubble at the end of the episode? It's cracking. Almost like she's trying to break out of it. 

When you take into consideration that weirdmaggedon is basically Bill using his twisted imagination to come up with horrific things to replace normal things in the world, it's only natural that he would be scare of Mabel messing up his plans because she is the QUEEN of using her imagination. He knows that if she were set free it would possibly be the end of his reign because she would certainly be able to dream up some way to defeat him. Because in a world where the imagination rules, Mable will always come out on top. 

Also the cryptogram could also be talking about the original Pines Twins as well. *clutches heart because I DON'T WANT ANY OF THEM TO DIE*

Specifically the line "Mabel's bubble is the most diabolical trap i've ever created!"
Also, take a look at this:

It's red and seems to be in a storage closet or something. This is the bubble i believe Mabel's in. It's more hidden away and smaller compared that giant pink one in the middle of the town.
Bill obviously wants something from Mabel. Maybe the bubble is not only a trap, but an incubator? You mentioned Mabel's imagination, which if used on Bill's side, could destroy everything. Remember what Mabel called herself?

Also remember quys, Journal 3 isn't gone forever. Stan photocopied the pages in Scary-oke

So the finale is going to be an hour long sometime next year. With alex's one tweet about somebody not surviving the season, I hope he means bill and not any of the other characters. I have not been this nervous over a series finale in forever. Man, am I gonna miss this show.