As others have said, Arena is likely some twist on a Horde mode. Which honestly could be interesting if done correctly. I'm thinking the challenge arenas in Borderlands 2 here as an example. Hoping it will have matchmaking if that's the case. I know folks wanted another raid, which I can understand, but let's wait for details before everyone freaks out. It could be fun and the rewards could "possibly" be good. Cautiously optimistic.