Dipper point of view
(They just defeated bill,but at what cost. As bills pyramid disappears around us I trip and fall. I can't pull myself up, curse my noodle arms!) As I lose my grip I wake up in my bed at the shack screaming. "What's wrong bro bro? Have a nightmare because it's the first night of summer at grunkle stans?" Questioned mabel
It's nothing. As I thought ( How did this happen?! Did the residual wierdness along with me getting hurt somehow send me to the beginning of summer? If it did I can stay a step ahead of bill! I can find that all knowing mailbox and ask for all the ways to defeat bill! Grunkle stan wouldn't have to lose his memory!)
This is just an idea of how to do a back to the start story for gravity falls. I never find any so I thought an idea like this could help some author start one.