I really like the stereotypical horror laboratory/graveyard theme, it's a neat match for Eggman's shenanigans. And the attack animations are pretty neat, even though there's not much of a reason use more than one melee attack at a time. Rouge's cartwheel kick is a pretty cool choice.
As for gameplay, I did get that fun feeling of exploring a stage and seeing all the silly doodads and finding the emerools. Although, as smooth as the gameplay felt, I did find it lacking in some places. I understand this is just a demo, though, so some of these complaints might already be on your todo and I wouldn't know. But still, I'd like to see this be improved/expanded, since treasure hunting stages were my favorite!
First, I have some complaints about the radar. Having the radar pick up on only one emerald like in SA2 isn't my preferred way to play the hunting stages. It restricts the player to only one section of the map, whereas I always thought these stages were about exploration. I think speedrunners would appreciate the radar picking up multiple emeralds, too.
Another radar complaint is that the color gradient going from light pink to blue and from blue to purple can look the same. I had trouble figuring out if I was getting closer or further to one emerald, the first big wtf I had. You should probably pick a different color that blends more distinguishably. In fact, find an alternative for color blindness as well; I'd suggest using something based on value instead of hue, or a meter/gauge?
My last complaint about the radar is that it's so far away from the center of the screen, arguably where the player wants to be looking. I've heard players mention that they get tunnel visioned on the radar, so they don't really pay attention to their surroundings. Although if you were to center it in the BotW style, as for how you would distinguish multiple emeralds, uhhh don't ask me...
As for the hunters, they feel sluggish and heavy here. I checked with SA2 right after playing to compare again, and I missed how light on their feet hunters are. They could jump high and move fast, but not like Sonic fast, just fast enough. The glide feels like it doesn't cover much ground, probably not helpful for when your levels will get more vertical. I misjudged how far a glide would take me over the toxic pools, and I was surprised I couldn't clear it all the way.
As for level design, this is a good start! Lock-and-key is an easy gimmick to start with, but I would request that it be made more clear that the doors were doors that can be opened. Maybe a fat keyhole texture? Colored locks on the doors? (btw maybe use shaped keys with color for the colorblind)
Anyway, I look forward to seeing this developed more. I don't think I've seen others tackle the treasure hunting gameplay yet, so I'm eager to see how it'd be iterated on ^_^