I think it depends on how the developers want the game.
If they want to keep it simple with one global rank, then it should be kept the same as it is in Arena 3 (the global rank should after all correspond to the best overall player).
I recommend having different specified powerup sets (gspeed, thin, rs etc) which have their own ranking while having the global rank as the average of them. People come to realize that they are better at a certain powerup setup than the others and would like to play that one more. Players would also master all of the items quicker if they could master/practice one setup at a time. Although, this can only be implemented when the game has more players and can afford to have different arenas.
As for teams, I think you should startup with no items at all and/or all items.