User blog comment:Master Sima Yi/Assassinews 19-03-'14 – Two Assassin's Creeds for 2014?: Unity and Comet/@comment-24760042-20140702114028/@comment-62.29.32.55-20140702140615

I don't think there will be optional objectives in Unity because mission structure has changed. Amancio said that there is a new type of mission which is the Adaptive Mission Mechanic and he gave an example where you're tailing a guy and if he sees you he starts to run and if you lose him you must relocate him etc... So their intention is to give a lot more freedom to the player and let us choose how we want to play so I think it's unlikely that we have mission constraints in Unity.