Ubisoft has stated that Assassin's Creed IV: Black Flag will have a "narrative connection", and a hidden Easter Egg will make reference to it, according to developer Ash Ismail.
Some possible games include perhaps Watch Dogs, which could be tied into the modern day Abstergo Corp, or Far Cry 3 (or 4?), what with the jungle settings in Black Flag's world.
Commence the speculation.
Ubisoft teasing easter egg in Assassin's Creed 4: Black Flag
18 June 2013 | By JonahFalcon