I wonder what the prizes will be for leagues. I'm hoping for something useful but not holding my breath.
An epic item would be a useless prize, since it's easy to get a few of those a day just opening boxes and doing missions.
Gold is really only useful if you have a backlog of items from before. Because the box costs scale so quickly, extra gold gold after you've opened 10-15 boxes doesn't take you too far.
Boost power would be decent if it was a lot, like 30k+, but I doubt they'll do that. If it's like 10k, that could easily be gotten from opening 3 boxes.
A legendary would be wonderful, but highly highly unlikely.
The best thing they could do would probably be a token box. That way it wouldn't be totally pointless like an epic, even if most of the time that's what it would be.