Hey, thanks for the feedback, really appreciate it!
I originally had the addition of customizable characters and a career + splitscreen mode in mind aswell, but as it proves making a such a (comparatively to the gamejam-version) big game is quit hard (also I'm a bad sprite artist myself).
Regarding the difficulty, I actually managed to implemented them! When selecting your opponent, the number of stars flashing up represents their skill-level (I had hoped it would be obvious). But again, making an AI that is actually fun to compete again in the long run proved really hard.
Nevertheless there will soon be a (small) update adding some more ambient sounds by John! Also I'm really glad to see there are people like you caring for this sort of game! Huge updates aren't likely to happen in the near future or even to the current game, but I'm definitely still willing to make a "bigger" tennis game sometime!