Nice idea, but then why bother having a player limit in an alliance if you can have one person leading multiple alliances from 1 account?
There is provision for more members in an alliance by using cp. I enjoyed being in one of the biggest alliances ever, but I can understand the reasons for limiting the amount of players in an alliance.
The more alliances you lead, the harder you will find time to actually play S.O. , you would end up becoming more of an admin than a player.