After some discussion on rgc I have some thoughts I would like to share.
It would be desirable to break the 12 player limit, as Blizzard itself has done recently. However, with more players, the first people to die would be waiting a substantial time to respawn, and deathmatch isn't nearly fun enough to be a reliable default. Many new players will not tolerate such long wait times and will leave. Standalone will not have the kiddy pool of bnet to house noobs and teach them the basic interface. It needs to have a mode that public players can find rewarding and entertaining.
Thus I propose the concept of persistent play. Upon death, the player instantly respawns into the next available arena (chosen by gold level, and rank seeding).
The excess players form a queue from which new arenas are spawned if need be, or recycled through old arenas as the round ends. (They could jump queue to a desired gold level, and have custom preset builds so they don't have to buy endlessly. For example one might want to play 20 rounds at 50 static gold [equivalent to league round 4], or if no such openings are available, jump to a 70 gold round. This also makes it much, much easier to train specific strategies.)
Instead of playing for an explicit game win, players will have the option to play for their career stats. Kids these days are stat and achievement whores, giving them a few cookies is the way to hook millenials.
Of course the tournament and league modes ought to be kept to accommodate the existing fan base. I know you are a long ways away from this, but it is never too early, or too late, to begin a serious conceptual discussion.
Next stop, ESPN.