I honestly think the replayability of Uncharted 4 is going to be off the charts based on what we're hearing. If ND has some extra time, it would be amazing if they could implement some of those game mods that were in Uncharted 2 (low grav, unlimited ammo, donut drake, etc). Using the low gravity mode allowed me to understand how good the draw distance was during the hotel level. When I was playing it, ended up blowing up a pot into pieces and kept using the dragon sniper to see if I could keep shooting one of the pieces further and further away while it was floating in the air. If this game ends up some how runnning at 60FPS, there's no way we're getting any of this though.
I'd also, love to see a horde style mode in Uncharted 4 online multiplayer. I wonder how much of this upgraded AI the PS4 could handle at once online. Maybe ND needs the "Power of the Cloud"