Defictionalization: Sony released a bluetooth headset styled after Snake's Solid Eye around the game's release, which was even compatible with the game itself and could be used to listen to Codec calls.
Talking to Himself: Averted. Being the first game since the MSX2 era to feature both, Solid Snake and the elder Big Boss, the latter's role is played by Richard Doyle (the voice of The Fury) in the English version, and in the Japanese version by Chikao Otsuka (Akio Otsuka's real life father).
What Could Have Been: The game had a bigger emphasis on the factional warfare between the PMC and whatever rebellion is in the area, and the war aspect was more played up, with the war afflicting the environment - and thus, places to hide, such as tanks blowing out buildings or the vehicle you're hiding in being driven off to fight. Enemies didn't even sound an alert when spotting you - they shouted to inflict Psych damage.
In an "MGO Eyes" video, it was revealed that Metal Gear Online had three additional maps that did not make it into the game: the first being "Desert Duel," the second being a recreation of the Town stage from Metal Gear Solid: Portable Ops that was never officially named, and the third is "Lethal Leviathan," which was actually the ship bow of Outer Haven. The first and third were eventually added to the game's arcade version, Metal Gear Arcade.
Not from the game itself, but there was originally going to be more novels besides the novelization for this game written by Project Itoh, specifically Snake Eater and Peace Walker. Obviously, these plans were dropped as a result of Project Itoh's death from cancer. Novelizations for these games were eventually made by other authors.
Originally, Johnny's grandfather (also named Johnny) was going to appear at the wedding, confirming that he did survive Metal Gear Solid 3 and Portable Ops. Unfortunately, this never made it past the concept art stage.
Writer Revolt: Originally, Kojima wanted to end the game with Snake and Otacon turning in to authorities only to be executed for being terrorists. His staff flat out refused to work on the game if it was to end that way.