This is not really my idea, but I think it would be really cool if you could work this out:

Mabel is NOT fine, is furious with Ford for taking her brother away from her. She goes back to Piedmond and misses Dipper really much. In Piedmond, when she's dreaming she gets contacted by Bill. He says he wants to help her. That he feels sorry for her. He tells her that he could help to get her brother back, but she has to help him in return. Being desperate, Mabel makes a deal. And then... everything goes wrong!

(LOL I'm not English, so excuse my bad grammar :/ )

I'm curious. Since Ford is gonna kick Stan out at the end of summer, where will he go? Will he use the money he made at the Shack to purchase a boat and sail the seas alone. What if Mabel went with him and they had their own adventures sailing the seas, and meeting Mermando and other ocean anomalies? Just a thought that popped out of my head. I would feel bad for Mabel if her adventures were over, and she had to go back to a boring normal school life. I wonder what happens next. Looking forward to more! XD

I like it so far, When U start on the next chapter, (idk) It might fast forward to the party and Dipper has to leave to help Ford And By the time he gets back, Mabel is nowhere to be seen

Ok yeah, I like this idea, despite the fact you are messing with the continuity of the series. But I have to admit I would ahve loved we had had more time in the aftermath of Dip&Mabvs the Future. So this is great for me.
So, I suggest you that you still continue with the Weirdmaggedon, in a way. Well, this could be another timeline in wich the Weirdmaggedon takes longer to arrive. Maybe because Bill didn't got to Blendin that easily. Anyway, you could have some more chapters of Mabel trying to get used to the idea of having to go back home without Dipper. Also, see how Dipper is considering the decision he has taken. And have Bill thinking how he can use his Blendin disguise to get inside the shack. You'll have to say if Ford and Dipper succed in patching the rift. If they do, I'm sure Ford have equipment below the shack that can be used to "liberate" the rift, so Bill will try to get down there.
You may make the story last for more days and even reach their birthday. How about unleashing the chaous of Weirdmaggedon on therr birthday?
So that's it, maybe you can do some of this.

We'll, teen angst. She begins to question her own self worth, and begins to doubt it- and then it deteriorates from the event. An event like this might trigger these thoughts... Like "did I do something wrong? Is Ford just better than me? Is there something wrong with me?" Then it might turn into a depression, which could lead to eating disorders, cutting, or other coping mechanisms. Maybe all of them, maybe not. Maybe even none. But Basically you chose a good plot point to work with.

Maybe something could happen to Mabel and then Dipper goes to see her and then he feels really guilty or something?

put the party and a lot of people come to say goodbay, it's your decision to put any character like pacifica, wendy, soos etc. and maybe you can put the last day that the twins are together good luck

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