1. Co-op Campaign. As we all know, the upcoming Assassins Creed Syndicate will have two Protagonists, so it might present an ideal opportunity for Ubi to implement co-op on a broad scale. We saw co-op (Albeit, poor executed) in Unity; but to be able to cooperate with a friend through an entire campaign is something I'd like to see. Sadly, it is not to be, as multiplayer has been nixed altogether.
2. The ability to use multiplayer characters in the singleplayer campaign. Comparatively few people liked the massive frusterating cluster-bleep that was the PvP multiplayer in AC III, Brotherhood, Revelations, and Black Flag. The reams of fanart throughout the internet, however, seem to indicate that a lot of people liked the playable characters within AC multiplayer. (My personal favorites were the Nightstalker and Preacher from AC III). So why not make it an option to play as some of them in singleplayer? Leave the dialogue and cutscenes unchanged, but let us play the missions as some of our favorite multiplayer characters. I mean, imagine playing through Revolutionary Paris as the Nightstalker, assassinating campaign targets with a meathook. Now there's an overpriced DLC I might buy. <3 Short of that, how about giving some of them mini-campaigns of their own?
3. Less Indestructible Assassins. Okay, I know I'm not alone on this one, at least. What's the point of having tons of cool stealthy gear, the ability to sneak and blend with the crowd, and a fricking "hidden" blade if you can just go into open combat and kill dozens of people anyway? I understand that Unity (which I have not played, due to price) attempted to address some of these issues, but this has been a long-standing problem in the series. I must confess that a little tiny bit of my enthusiasm for completing AC III died when I saw that Conner could absorb four musket shots to the chest with barely a wobble.
Well, there's my two cents. Post your own AC ideas that will never happen.