I just found these theories on the web so i will list them.

Gideon Works with the Government Agents

Mabel is Transgender

Mabel's Future Disappearance

Mabel's Paranormal Twin

Pacifica's Memories Were Wiped

Dipper and Mabel Will Drift Apart

Pacifica Was Adopted

Robbie Was Possessed

Dipper is a transgender

Dipper Is Possessed by Bill Still