It would be an intriguing idea as an event maybe. But then on the other hand you would need 3 side slots (or 4 with Wu Lin) and so far every game mode had only 2 sides. Not sure how complicated that would be for the developers but I could imagine that it would be a little too complicated for them.
Besides, what you more or less gain from that mode would be the brawl experience if you play it like it's meant to be. It is as mentioned above a great training for dominion gank situations (how to work together well and how to defend against multiple opponents and adjust strategies on the fly)
But, in general I think that once the fundamentally mechanics are established we would have really cool opportunities for more intriguing game modes where faction teams fight each other.
As Maxime said this has been suggested many times. I've compiled a list of most suggested game modes for this game. Check it out: https://forums.ubi.com/showthread.ph...=ultimate+list