Also, MGS Raiden is an unlockable costume in Brotherhood.
Also in Brotherhood, when you first meet Leonardo you get a mission to find out where a noise is coming from. After going down some stairs you find a cardboard box in a corner.
Cash Cow Franchise: Ubisoft is doing this series every year along with many spinoffs, comic books, and maybe even kids toys.
Celebrity Voice Actor: Mostly averted. The only celebrity voice actors usually only show up in the "Modern Day" settings of the series, and even then, once Kristen Bell left (perhaps not coincidentally after her mainstream breakout role in Forgetting Sarah Marshall), the celebrities are probably only recognizable to Star Trek fans (John de Lancie) or fans of British humorists (Danny Wallace).
As time has gone on, more recognizable celebrity voices have started to show up, but not there still has yet to be any A-level celebrity guest voices in the series.
Fandom Nod: Brotherhood has one early on in regards to being able to relive memories, which was in 1, but absent in 2. When Desmond is talking to Rebecca about new features, Shaun pipes up with "Is he gonna be able to relive memories this time?"
For Black Flag, it's Pirate's Creed or Grand Theft Man O'War
What Could Have Been: As per Nolan North in a 2015 discussion, the initial idea for the series was that Desmond would eventually appear in a Grand Finale as the ultimate Assassin after imbibing the abilities of all his ancestors and that the finale would have had Desmond going back and forth in time as he becomes an Assassin in the present day. This idea had to be abandoned when the writers realized that they didn't like the direction Desmond's story and characterization were headed and they couldn't figure out a way to fix it, leading to the setting shift beginning in Black Flag.