The game play is a riot and the graphics are amazing.
There are a lot of areas in Dying Light that look like they should be open to exploring but aren't. Maybe you could expand on these parts and map out the island just by adding additional cities and country side areas while keeping the original cities and country side intact. Then you could have add 2 seater gyro-copters to get around the island. That would be a blast using both parkour in the cities, the dune buggies in the outer areas, and the gyro-copters to get around the island. Coupled with new mutations (maybe ones that also fly to harass the copters).
My friends and I are hoping that in DL2 you'll allow more players then 4, maybe 6 or even 8 players to work in co-op. You could set it up so that certain missions require actions done at or near the same time in separate locations that would make the team of 8 to split up into 2 or even 3 groups to accomplish it.