The Rules of Tennis state that you aren’t allowed to change the tension of your racket strings whilst a point is being played. (Lower tensions generally provide more power and less impact to the arm: higher tensions offer more control and better spin).
It seems, at least until some official body rules against it, that it’s perfectly ok to change one’s racket tension before every point.
In a game that’s constantly being accused of being boring because of the dominance of fast serves, it would certainly increase audience interest if players took advantage of this and attempted to anticipate the optimal string tension for a forthcoming point (based on eg how well they are currently returning serve or in attempts to protect a wrist injury).
Today’s invention is a screw-driven wedge device in the handle of a racket that would, without changing the shape of the head, boost or decrease the tension in the strings. This could be made operable without showing an opponent any new setting which had been selected.