While we still don't have an official confirmation that Gearbox is, indeed, developing a sequel to their popular Diablo-inspired shooter/RPG Borderlands, I don't think it's too much of a stretch to simply assume that Borderlands 2 is in the works. OXCGN doesn't think so either, as they've penned a new editorial that breaks down some of the original game's flaws and how improving upon them will help the sequel be even more wildly successful than its predecessor.
Another major letdown was the fact that Pandora simply had no incentives to go out and explore! The world is big enough as it is, but every location would be ventured to eventually if you did every quest.
That's not exploration; that's being linear.
Rarely were there places that quests didn't already take you to begin with. There is plenty of room to add in random encounters, unmarked quests, and even the possibility of areas that can lead to massive profit in terms of loot.
Add in a nice little boss with his/her own freeze-frame moment and caption, and you've got yourself a successful side-mission system.
But in no way do I want it to be limited solely to that! Add entire camps, bases, even cities that you wouldn't normally go to! Heck, even add a whole side story as yet another incentive to take a break from the overarching storyline.