Improving the Experience
The reason I hesitated to write about improving the experience is because thoughts were not exactly forthcoming. At a glance, AC2 has fundamental flaws working against story immersion and core gameplay, and the game is just so large and complicated.
The problem with gameplay is that the core of the experience is being an assassin, but I put far too little thought into the preparation or act of it.
Arguably, murdering someone should be a simple matter for someone of Ezio's caliber. Making the act of killing simple is not necessarily a mistake given the character, at least once he reaches maturity. (Did anyone else find it weird how quickly he became the de facto master assassin?) So should this have more meat mechanically? Perhaps less so in action and more in the systems that surround it. The truth is, the mystique of the assassin is often not met because there are no systems that motivate stealth gameplay over running-and-gunning. Oddly, the opposite is sometimes true (e.g., you have to spend money to get NPCs to distract guards, while murdering them all in plain sight and robbing a bank is faster and less costly). And this is not necessarily an issue of the combat system. Combat is a bit boring but I disassociate it with assassination, and is probably not a core enough feature to spend a lot more energy on.
So what about preparation? In both Assassin's Creeds, assassination involved little more than starting up the latest mission, going to a location, chasing down my target, and running away recklessly. The issue with this lack of strategy is that I hardly see the reason to be stealthy, subtle, or wait time to assassinate anyone, and it highlights that most of what I'm doing between major plot points is just nonsense. But the problem with planning is that immersion snaps when plans fail and you redo missions, with all of the pieces set right back in place again (and making this dynamic is extremely difficult).
The solution, then, may have less to do with adjusting the core mechanic. More can and probably should be done to motivate actions that embrace the mystique of the assassin (stealthy murder) over the smack-and-go actions that you default to without consequence. But a more effective approach to improve immersion may simply be to improve the reasons behind the busywork leading up to each kill.
This means story. More on that next.