What do you think the new mode will be about?
I think it may be like TFotG but you get favor by killing random enemies around the map and, if there is actually another team, also get favor by killing them. I'll wait for the final game to test this surprise, I'm sure this will be awesome.
Also if the devs have already create a game mode that have enemies from the SP campaign, why not making a horde mode? The only thing left for devs to do is a HUGE map(know it's not easy to make, but I'll totally agree if they release this as a DLC)
I think it will deal with teams and AI monsters. If you look at the "Big Reveal" Video, it shows a two person Cyclops kill before entering the arena. I can really see this being a multiplayer mode by itself. Maybe like a horde mode where you have waves of monsters attacking your team with the difficulty increasing with each wave.