Hi Manu,
Here's an idea about shot power, especially in reaction to the new talents related to accel shots. I liked several of the previous talents you created because they were complementary to stats and game - there's no stat/button to be a good slider, for instance. But regarding shot power, and accel power talking about latest talents, I think there could be a system to set more precisely the power one aims to put in its strike. This could also be the case on serve, so partly also answering to your post about a new serve system.
The idea is simple: link power of the shot with preparation time. It is already the case with accuracy, and it is some sound thing. But the thing is that sometimes you can prepare for long, be well positioned etc and send a "weak" accel - or at least weak in comparison with the time you had to prepare. So I think link linearly the preparation time with the power of the shot could be good, both for regular strikes (the more time you have, the harder you can hit it with some accuracy) and also for serve (this time the holding time not linked with accuracy, you just decide to send a more or less hard serve). So here's the first idea, linear link (or close to linear relationship) between holding time and shot power.
An additional thing is that, even if you can prepare your shot with a lot of time, if you hit it super hard you will obviously lose accuracy starting at some point (going into the net or behind the baseline). So if implementing a link between strength and button holding time, also decrease accuracy when shot power becomes very high ( = when button holding time becomes very long).
Hence, with these two things if you hold the button for short, you have not super accurate weak shot, if you hold it for medium, you have accurate good shot, and if you hold it for long, you have less accurate (though maybe not as if you hold for short time) but hard shot. So that there would be both the preparation time that would matter but also the timing (of button holding): it is better to hold for some time before shot, but without ensuring holding for extremely long time ends in extremely accurate shot, you would have to also decide when to *start* holding given the shot you want to send back.
Hope this can give you ideas, the "power" decision thing may be a bit lacking in the game at the moment, and implementing it with also a bit of timing thing could be, IMHO, a great thing.
All the best!