1) more interesting things to possess i feel that possession was limited there was no flying creatures you could control like flies or birds, also i felt it was stupid how you could jump or use enemy weapons while in their body
2) better weapons maybe a rifle weapon thats stronger but slower or perhaps a stun baton for non lethal melee combat (like a low level ark pilon something like a baton was shown in esmond borrows story) flash bang grenades to cause heavy distractions, perhaps a frenzy dart that causes a npc to violently try to kill whoever is near by (and in turn be attacked)
3) perhaps more free roam, be able to explore more more side quests maybe change the chaos system to have a medium (killed a small but noticable amount of people) or maybe neurtal ,high, very high, low, and very low. more endings then just a good and bad ending
4) more unique powers 2 i would like to see is the assassin verision of blink and their telekinetic pull (which looks like a reverse windblast sort of) maybe a 3rd rank of possession that lets you use the possessed npc's weapons
5) better story line, dont make the first missions involve taking down extremly important characters (ie high overseer camble is highly important there should have been a few missions small members of parilment to take down before him) maybe introduce a new enemy thats a threat to possibly even the outsider, a redeemed or more corrupt daud assassin depending on how corvo dealt with him