I plan co-op to be just like single player but just another 1 person to help you fight enemies! and i may add somethings that aren't in single player. EX: Bosses, Characters, etc. I will update you guys when i have more ideas on this topic.
For Multi-Player i plan to make it where you can fight someone else in pvp, i may also add Team Bosses that only a certain amount of people can face. I don't know if i should add character customization (kinda hard to make). and you guys will see what happens in the future.
** I will open a topic in the forums tab on the Multi-Player Topic**