-Allow offline (or online) splitscreen play (co-op/campaign)
-Balance the multiplayer boosters as much as possible
-Remove Situational Awareness
-Allow regional matchmaking, thus preventing major lag in some cases
-Fix cover system so you don't accidentally cover when you want to roll...
-Beta test and hell out of the multiplayer. When everyone likes what they're playing, DO NOT change anything drastic (ex. 1.04 to 1.05)
-Add more puzzles into the game
-Improve stealth elements
-Add more variety in villains
-Add more screentime for Sully (one of the best side-characters in any videogame)!
-Allow recently-logged on friends to spectate other friends' current match (real-time)
-Improve clan system
-Expand team size?
-Add in more gamemodes for specifics (ex. Sniper Rifle/Pistole Deathmatch)
Regional Matchmaking, more puzzles & more/improved stealth sequences. Also, the larger player compacity seems like a great idea if the ma[s are as big as The Flooded Ruins, The High Rise, etc. Maybe more weapons too?