Mabel not wanting to leave the bubble because it's her ideal summer seems likely, but I don't want that happen. I want the bubble to be a nightmare on the inside. I want to see Mabel's insecurities and fears and just her horrible experiences alive in there. Mabel isn't all glitter and sweaters, we've established that since NWHS, but I know there's more to her. And I want everyone else to see as well how much she's been bottling up.

True, though an ideal summer also ties into her insecurities and fears… She wants it because of that. Still, I’m also fine with if it’s her mindscape or maybe the bubble contains idealism AND horror. (Heck, “uncanny idealism” as I can call it, will be just as if not more interesting! Things seeming fine and ideal at first, but really it’s just creepy and unsettling.)

With the way the bubble seems so giant and corrupt before Dipper, Wendy, and Soos enter though… I wouldn’t be surprised if it’s just nightmares on the inside as well.

Also, when things get bottled up and such, it CAN be represented in a visual like that? At least, I imagine bottled fears and other emotions could have a visual construct similar to Mabel’s bubble.

Like many have noticed, Mabel’s bubble indeed starts out VERY small and clean and such (but still creepy and dreadful in appearance). Then, 3 days later in the episode, it’s shown to be ENORMOUS and CRACKING everywhere! It literally reminds me of an erupting volcano or something. Y’know, like with the glowing lava cracks and such?

Whether that’s caused by Bill himself, or the bubble has a connection with Mabel’s emotions, or something… It’s still pretty darn scary.