Why not just let coaches challenge whatever they want - and if they lose they lose?

Coaches get two flags, and if they use them and win the challenges, they get another. A coach might challenge every play in the last two minutes of a half, like getting unlimited time outs, so it goes to booth review without play stopping most of the time. I do believe some penalties could be challenged, but mostly they're judgment calls anyway, so let it go. You play well enough to overcome bad calls, or you don't. Spotting the ball should be reviewed. I'm somewhat satisfied with the game as is, but there is room for improvement, as soon as they figure out how.