Instead asking for more classes i would like to ask to make existing ones more distinct.Making each class more unique and different to play could vastly improve coop (to be actual coop, not just few people playing in the same dungeon next to each other). Each character could have some special trait like: paladin being more resilient to arrows thanks to his shield or warlock to poison thanks to his poision training (or wickedness). Also new more team-work oriented abilities like palading being able to taunt enemies or mage summon some monsters to help others or thief drop some smoke bomb to help with escape (something like priest healing beam but more of it ).
If monsters would be samrter and used different attack instead of just swarming then gameplay would also be more tactical and in need of more cooperation (and i dont necessarily ask for it as it would require lot of work, just point it out).Or maybe some simple leveling (always better to kill monsters when there is even small reward ) like random experience drops. Trine did it in nice simple way that gave you some choice in a way you wanted to play with each character.Tell me what you think, or if you agree with anything i wrote.