I like the pinball-tennis idea :D I've considered various pinball-mixed-with-other-genres too, there's a lot of fun possibilities when you throw in ball mechanics :P Maybe I can make a prototype of such a concept after this game :)
I'm really not looking forward to making net bulge mechanics, sounds hard :P BUT I agree some effect is needed! How about fireworks/confetti coming from the goal when you score?
Cool that you like Godot, I hear more people being positive about it! I'm coming from C++ so C# is probably better for me; I'm always fighting with Python :P But I guess it really depends where you are coming from and what you are used to :)