In the final glitched memory in the castle, there is a courtyard with two children. They will say their names are Arno and Elyse, which are the names of the main characters in Assassin's Creed Unity.
Other Ubisoft Games
If you look around in the offices located near the Animus, you'll find various Ubisoft games displayed, such as FarCry and Splinter Cell.
The Headless Horseman
Visit the town of Sleepy Hollow and look for the church. There, you should find the Headless Horseman, famous from his appearance in the classic short story by Washington Irving. He carries an ax, and is a lethal foe who won't die, but you can use a sleep grenade to put him to sleep.