after some thought, I think the context-sensitive action button might not be really needed.
Here its possible uses I had thought of in the past, and why there are not needed anymore :
1) Sprint : the player now always accelerates his run at his max so this wouldn't bring much, especially for people having a controller with an analog stick that allows them to run slower if wanted
2) Drive volley : now it almost never triggers anymore, wanted or not, and using the topspin strike button to do one would be good enough if I ever want the possibility to exist
3) Turn around the forehand to do a backhand (and vice-versa) : this would avoid a silly transition animation, but gameplay-wise, it doesn't bring anything
Moreover, it could lead to deep gameplay changes that may upset the people who already bought & played the game for a long time (eg: the sprint), so it's risky to add it now.

If you'd still like to see a context-sensitive action button, please let me know what you'd like it to do.
EDIT:
4) Tell our doubles partner to play the ball (can be useful in some extreme cases where the ball is on our side, but we cannot reach it while our partner could)