I, for one, think Aerial Garden's concept and theme is worth salvaging. It's got the "floating ancient ruins" thing going on, which we've seen in Sonic before, but AGZ still stands out from zones like Sky Sanctuary because of the "garden" part, which is at least visible in the current design with all the grass in the outdoor sections. Plus, AGZ is the game's only sky level as of now, and even once Grand Eggship gets introduced, a ruins/nature sky level like AGZ would feel plenty distinct from an airship level like GEZ. I also like some components of AGZ's aesthetics, like the sky texture introduced in 2.1 (though I know the devs would probably want to turn it into an even better-looking skybox), and the music, especially after its 2.2 revision.
That said, I think AGZ's map itself could use a thorough transformation or complete remake - not just to get rid of the more frustrating platforming choices and secrets hidden behind unfun button hunts, but to help the zone live up to its full thematic potential. I'd love to see AGZ lean more into its distinctive "garden" theme - that is, the fact that whatever ancient civilization built this place was intentionally growing plant life there. Imagine if AGZ had larger and more impressive flora than grass, small flowers, and the occasional seaweed (EDIT: and decorational bushes). The plants could be used as platforming gimmicks (or just straight-up platforms), and would give players more to look at besides the endless walls of carved stone. If AGZ is going to continue to be a long level, I also think it would be cool if the time of day changed as you progressed through it. By the end of the level, it could be straight-up nighttime, making the final sections evoke the Starlight Palace from which they were lifted.
As for the Hive Elementals and Bumblebores, I'll echo a complaint from the original Mystic thread:
The bees are meant to attack from above. They position themselves above the player, lock in place with a sound cue, and then after a short delay, slam down. This attack is telegraphed, and provides players sufficient time to get out of the way.
However, the bees can also just slam into the player from the side, if they happen to be on the same vertical plane. And if this happens, there is practically no preparing for it. They can come from outside the camera's field of view and strike quicker than a player can reasonably react. The only way to guarantee this never happens is just to never be out of ball form, which is easier said than done.
Usually what causes it is any situation that involves vertical traversal, because while the bees usually remain above the player so they can divebomb down, if the player moves up (such as to reach a higher platform) the bees will sometimes hit the player directly while attempting to move into position above the player to do their normal attack.
To fix this issue, perhaps Bumblebores could be made incapable of dealing collision damage until they enter their deliberate attack phase?
Finally, if we're uncomfortable keeping around a level so closely tied to Mystic, we could always completely remake AGZ while keeping the general theme, and then rename the level so it no longer directly references Mystic's work. Evolve SRB2's "AGZ" into a separate thing from Mystic Realm's AGZ, and give Mystic credit for the inspiration, but the inspiration only.