I was just wondering if there is anything like a special item or upgrade you get for 100% the game, haven't finished the game fully yet so no spoilers please!
Completing the Abstergo challenges unlocks cheats (unlimited ammo, invincible, always drunk, etc). As far as I know there aren't any special outfits or weapons for getting 100%, aside from the ones you would pick up along the way from finding collectibles and completing side quests.
UPDATE: You receive a multiplayer emblem when you complete 100% of the challenges. Thank you Nolanar.