Idea - 'Challenger' teams
In order to scrim with the challenger team, maybe you have to use your daily sandbox matches or coach's training points - I'm not sure but you'd need to put some sort of tradeoff in so that you are losing something in order to gain skill levels in your prospects.
TL;DR: Possibly establish a 'Challenger Team' tab where you can train and test out prospects acquired in scouting and academy.