They've added way more hooks into the software to make DLC bad ass. BL1 didn't even start with any way to add DLC content, they had to patch that capability in after the fact. New characters as well as new areas may be what we see this time, and they're well aware of what worked and what didn't work content-wise last time. No more Moxxi crap as a DLC pack.
It's completely possible that if they really add big areas as well as new playable characters, they won't be coming out at the $10 DLC price point, probably closer to $15 or $20, which would make the season pass more of a must buy.