I've been playing with Jade mod and it has a nice feature to make tooltip edges square. Also, it would be nice to be able to use a ready-made texture for the background (for example, to be able to use a HD background).

I thought a little about the texture for the background. Perhaps it would be logical to add the setting "Use a separate texture for the background" to the mod's config, and if there is "assets/waila/textures/background.png" in the available resource packs, then use it, otherwise use the built-in one.