...walk rule in baseball. No need to fair catch--just indicate you wish to do it, and the ball will be marked at a yard line commensurate with where it first hits the ground on the kick. Or just have the team "receiving" start on their own 25; for, if the concern is that there is an inordinate level of injury on kickoffs, why use half measures?
This preserves the onside kick (both expected and surprise)