I'm honestly expecting some more futuristic weapons and overhauled maps, but other than that I'm not really sure. The game already has a lot of the things that I love about games of this genre.
Some sort of pets system. I understand Mordecai's whole deal was that he was the one with the eagle, but I still think everyone should be able to have some sort of friendly helper. Killing bandits can get pretty lonely
I don't know if it already exists or not, but because BL loves to have weird weapons where the actual bullets travel in unique ways maybe there could be a laser-tagging system for aerial strikes or to have drills shoot out from the ground where the laser is pointed.
Ooh, that sound amazing. I only started playing the game yesterday so I maybe talking nonsense here, but I'm not too sure about @FuZyOn 's idea about futuristic weapons. I feel like Borderlands already has a very nice and unique setting and there's not need to make it anymore unique than it already is.
I would love to see the further exploration of the sci-fi elements and themes that have appeared so far, and maybe more of whatever type of alien the destroyer was. I think the Pre-sequel had it right in terms of expanding on the futuristic game-play elements.