I think leagues should open up to either 6v6's or 8v8's and eventually drop which ever doesn't get enough support. But 8v8 would mean you would have to have your entire team of 8 online, instead of 6. If you had a full roster of 10 - 12 people, and you play 6v6 you would most likely always have at LEAST 6 players on to scrimmage.
But I'm wondering if you can actually do this, bots might always be put into the game to substitute your missing two players. I don't think it will be a problem for PC gamers but for people who are playing gamebattles on 360 or PS3, it's going to be a bit trickier. :\
We'll see what's up after release.