Keep in mind in s3/s4 for arena, most successful multibox arena teams were (1) in the 5s bracket, (2) consisted of 2 players, a mb controlling 4 shamans and a second player controlling a healer, there were maybe 3-5 teams controlled by 1 player that hit high arena ranking (1900+) and virtually no multiboxer was having success after patch 3.0. Its really hard to tell at this point what teams will do great in season 5 -- could be a few DKs, maybe hunters, maybe shamans. The later a season gets the harder it gets since your new team is going to be undergeared compared to other teams, something else to keep in mind.

Its still a hell of a lot of fun though, especially when you start working out strats for countering other powerful teams.