Asscreed has always had a solid foundation, but absolutely botched execution. I think if they merged the flashy swordplay, escape sequences and acrobatic movement with Hitman-esque social stealth and open-ended missions you would have a winner. Imagine a level with a Medici walking around inspecting a cathedral under construction, and there are a ton of ways to make sure he "accidentally" dies. Maybe have multiple characters where one character is the fighting expert with execution counters and disarms, but can't do the acrobatic stuff - and another who is great at parkour but can't do crowd stealth very well.