I'd like to hear some ideas about what you could challenge and what you should not be able to challenge.
I was watching a game where the player made a great move took a dribble two huge steps and made a difficult running hook shot and was called for a travel. The player was upset, it clearly was not a walk. You should be able to review that at the coaches request.
It would slow the game down a little, but only two per game not a big deal they already review plays now, let's give the coaches some more power it may make them less crazy as well.