It is clear that the rules today for injury timeouts can be abused from players intent on halting the opponent's momentum. Cramps, blisters, muscle soreness are not injuries and a timeout for a player to address those penalizes the opponent. One way to address the issue is to allow for medical timeouts at the end of a set. Any other request for a timeout during the set will incur a game penalty, unless it is related to an actual injury. This way injury timeouts become costly, and the opponent who did not get the timeout gets compensated for his opponent's timeout.
This approach doesn't sound too bad. It would make sense to introduce a sort of penalizing, but it would have to be with any injury time-out occuring, no matter if the injury is legit or not. Otherwise you're running into troubles again to judge what qualifies as a "real" injury or reason, and what not.
Of course experienced trainers and doctors can feel most muscle strains and other problems. So this could be a solution. But the chair umpire is unable to judge or detect that properly more often than not.