I was thinking for those with non-permanent transformations once your powerlevel is knocked to base or below you automatically revert back to base. Vegeta does it all the time when he gets owned...Poor Vegeta.

Also this is another addition to the War Cry idea but have it where you dont need to be right next to your guild member. Make another where you can lend someone a percentage of your power. Giving them that percentage of your stats for a few minutes.