Recently Dungeon Defenders on Steam released a new piece of DLC called the "Hero Pack." The $5 content adds four new characters to the game - a female Squire (the Countess), female Apprentice (the Adept), male Hunter (the Ranger) and female Monk (the Initiate). These aren't just character skin swaps, but actual class changes and tweaks depending on which version of the character you choose.
If you purchased this content, now because of the availability of 8 different characters, there's a brand new 8 player co-op mode. The mode is called "Uber Monster Fest" and it's the same style of play as the one already found in the game - but the difficulty has been ramped up for the increase in players. I imagine it's absolute chaos.
There's no word if the console version of Dungeon Defenders will see this same content.