I'm considering removing the 3 buttons for normal/safe/strong serve by only 1 serve button ; then when we serve, a charging bar will appear to decide the strength of the serve. The stronger the serve, the less chance it'll have to be in, exactly like currently, except than having only 3 steps, we'll get a lot more.
It wouldn't be dexterity-based, it's only to give choice, not a timing to succeed.
Optionally, the serve chance would appear next to the charging bar.
I would likely put 3 bigger zones where the power would 0% ( = legacy safe serve), 50% (legacy normal), and 100% (legacy strong), and between them the charge would smoothly raise.
Overwell, this would bring more variety in the serve speed.
I think the main downside is that newcomers will be convinced it's a timing bar like in other tennis games...

It could also be a bit cumbersome to charge every 1st serve.
Would you like to see that in the game ?