Since in general I don't have a lot of theories ,only one has been debunked. I thought Brulee's house would be involved in how Luffy would escape the mirror world. Never came to fruition .
One that is a trivia and not so a theory and I am is that the island of Justice is always illuminated through the awakened usage of Kizaru's fruit,from a previous user.
Another one that would most likely be debunked, but it's funny and I am Firm believer is that BB eating WB testicles,enabled him to gain his gura powers
.Got into the theory even more after BM ate Carmel.