I think a reasonable compromise, in addition to what Woodrow proposes, is to allow the opponent to have coaching when someone takes an MTO. Like it or not, an MTO will throw off the opponent whether or not legitimate, so this will help. There is coaching allowed if they go off court (rain breaks) so why not this?
Giving the opponent a reasonable tactical advantage (like coaching) will serve as stronger deterrent than just waiting till one's serve. If someone is bent on using the MTO as a dirty tactic, waiting 1 more game is not a big deal. Also, I am not sure the serve thing is very relevant on the WTA where many players don't even count on holding serve.
Also limit the time. As it is, nothing can be done in 5 min (treat a locked rib in 3 min
give me a break), so why not make it brief?
No back-to-back MTOs. If you want one, give up a game or a point a minute. Bad luck if it is legitimate. I find it hard to believe that top-class athletes would be playing with (or have them crop up during a match) multiple conditions when they are so fit and well-conditioned and travel with trainers and physios and what not.