MGS Raiden is an unlockable costume in Brotherhood.
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.
This has caused the series, and Unity in particular, to suffer in quality. Thus Ubisoft has decided to put in a year break dedicated to developing the game following Syndicate to avoid another disaster and at the same time avoid over exposure with the movie's release.
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.
The E3 trailer for the first game shows Altaïr assassinating a Templar who is about to hang someone, shooting his bodyguard with a crossbow on the way. Nothing of the sort happens in the game, and crossbows were not available until two games later, in Brotherhood.
The trailer for AC II shows Ezio chasing and shooting a masked Templar with his Hidden Gun during Venezia's Carnevale. Said Templar only appears in multiplayer, nowhere to be seen in the actual game, although a similar assassination does happen, but with a different victim and very different circumstances.
Brotherhood's trailer shows Ezio challenging and fighting Cesare head-on in Rome; no such scene occurs in the game, and when you do fight Cesare, your Assassin Recruits are not available to help, and it doesn't occur in Rome.
Averted with Revelations, whose trailer is the only one to show a canonical scene; in this case it doubles as the game's introduction.
In the AC III trailer, Connor charges a British firing line head-on and assassinates their commander. This appears to be based on an early draft of the Battle of Bunker Hill, but trying to complete that mission in the fashion Connor did in the trailer is impossible.