I noticed that in the beta 1 arena you always HAVE to play 3v3, if you're with 4 people both teams HAVE to pick an AI for in their team, but they can't just choose to leave one spot open, to make it a 2v2 battle. I think that if this function was implemented you could have more different experiences on one map, 1v1 anyone?
I know many people will just fill all spots with AI's anyway but that's ok, you don't HAVE to leave those spots empty, I just think you should have the choise to do so.