To better differentiate my '20XX' set, I injected custom music into my Melee ISO's .DOL. (Don't worry, It's not disruptive or annoying.) The music injected to the game lacks loop points since I don't really know how to add those in right now, but when I do I'll edit the song .HPSs. The music injected automatically loops when it reaches the end of the track. Almost every commonly played stage has modified music.
Regarding modded stages, I don't have any yet (Aside for the AR code modifications which include No lava/acid on Brinstar, No Switches or Bullet bills on Peach's Castle, No Arwings on Great Fox, No Gun on Great Fox, No blocks fall on Green Greens).
No legal stages are modified in any way (This means Flyguys are still on Yoshi's and the wind is still on Dreamland). Later, I may be modding Green Greens and Fourside to be their 'SD Remix' counterparts for fun.
When you hear this song on the menu, you'll know you are at the right set:
As a reminder, this setup is not for Melee SD Remix, and is only for the Melee 20XX pack. It does not contain modified characters in any shape or form.