You know, I have an idea about the transparency.
I was thinking that if we change the colour of the bg in the main menu (The original bg)
We can also change the transparency by creating a picture to use in-game, done so by having a transparency map which is a picture that is done in a way where white represents 100% opacity, and black represents 0% opacity. As for grey, 50% or so...
With this, the menu visualization will have lots more of customization and mixing of different pictures, although, if you have supporter and used this, your picture can be the one where the transparency map is applied on or the background of the original osu! background which can also be done through toggle.
I don't know if you get what I mean so I will elaborate further if nobody understood o.o