I would love to see:
-New Characters
-New Dungeons
-New Loot
-New Game +
-Harder Difficulty (more mobs, more minibosses, and insane loot)
-Ranked Endless Battle (constant waves of enemies, mini-bosses, and bosses) One room endless enemies play till you die with just yourself or friends with an online ranking.
-Increased Level Cap (50 or higher)
-New Skills
-New Talents
-Ability to respec proficiencies (Limit one per playthrough to avoid abuse)
This is my favorite game I've played in a while and I love the fact I can game with friends in a drop in drop out scenario (Kinda like life friend drops in while I'm playing and they can pick up the controller and start playing with me). So yeah I know I'm not alone in saying I'd pay for more content for something which I enjoy this much.