The upcoming Digistruct Peak area sends you on a mission from Patricia Tannis, in which you make your way through a series of Borderlands 2 enemies. It's balanced for four players, so not the sort of thing you want to take on solo. The enemy classes will grow progressively stronger, and then become more randomized as you gain new "Overpower" levels.
Though the new level cap will be set at 72, the Overpower levels let you go past that number. If you enter a special arena at max level, you'll be able to unlock those levels starting at Overpower level 1. Then, when you host the game with friends, you can set which Overpower level you want to play at. That will give you access to monsters and loot up to level 80.
Gearbox explained to Digital Trends that it is using this method, rather than a full unlock of level 80, because it is running out of skill points for players. Instead of letting players unlock every ability, Gearbox still wants to keep skill trees diverse.