My favourite old theory was the T.H.A.N.O.S. theory where each letter was the starting letter of where each stone was hidden in the MCU.

T. Tesseract

H. Heimdel

A. Aether

N. Necklace

O. Orb

S. Scepter

Of course this theory was officially debunked but back in the day people had a lot of fun theorizing how to make the acronym fit.