I think there won't be any human AI that fight alongside you, unless it's a specific mission where you have to go save someone, or escort someone. I believe you'll be able to take some missions, or your approach in the real world how you want it. So weather that be stealth or gunz blazing. However, it would be wise to do things some ways in certain situations.
And of course there will be huge awesome vistas in the game, epic landscapes, epic architecture.
Assassinations however, I don't know, I don't think they'll be in the game.