There's a few things to collect and a couple epilogue cutscenes. Other than that, you're trying for full synchs that you may have missed/doing side missions that you didn't do before. --- This ain't no place for no hero.... New York Giants: 6-4 Next: Bye
After completion, you can complete all the Kidd, Homestead, Privateer, Frontiersman and Hunter missions you haven't done, plus there is something called 'Syncing' which involves hacking the Animus.
There's a few things to do, but there's not a great deal of after-story content, I must admit. That's rather something most if not all the AC games have suffered from, though. --- "I love finding threads like these, it's like walking outside and finding the city's been bombed while you were sleeping." - Blighboy