From Sonic Retro
Sonic Advance had gone through several changes over its development process. There are several rejected ideas and other things that were left on the cutting room floor. The following details several of these things.
Sonic the Hedgehog Advance was shown at E3 2001 but was not playable. A short video showing Sonic in a prototype Neo Green Hill Zone was released by Sega alongside a handful of screenshots.
Neo Green Hill Zone
In this screenshot numbers appear top right hand corner, possibly symbolising a debug mode similar to the Sega Mega Drive games.
Casino Paradise Zone
Egg Rocket Zone