NG+ would probably have more replay value then being able to look at a different character or having 10 more assassin contracts. Now it's time for the what if game. What if the skins look stupid? Or what if the assassin contracts are like they were in ACB?
The difficulty option is one of the things I hate the most in videogames. Please don't include that. At least, if you include them there anyway, don't make any trophy or achievement related to the difficulties like other games do.