100% Requires:
All Main Story Mission w/ their bonus objectives
All Murder Mystery and Franciade side stories
All Outpost missions
Both Co-op missions
All collectibles (Napoleon bicorns, chests, artifacts)
All Viewpoints
And all of Suger's Legacy riddles
You can check on your progress with the progress tracker from the pause menu. The collectibles have to be found the old fashioned way as well; the map you could buy from the eStore only worked for the main game, so make sure to spam eagle vision at all times while in or out of missions to hopefully trigger them all on the map. The underground districts shouldn't pose a problem here, but if you are having trouble my best advice is run along the outside edge of the map, a few bicorns and chests are hidden along there.
Note: The co-op missions are only required to be played once each; getting all rewards is not necessary.