Just some possible AU that's completely unrelated to the Sonic series. Feel free to suggest ideas, if you wish.
Note that this is an incomplete blog full of ideas.
Unnamed demon tree
This one would be an expy of the Saigyou Ayakashi, the youkai cherry blossom tree from the Touhou series (which I never played, mind you).
A full bloom of such a tree would be very bad, so it has a extremely powerful seal preventing it from fully blooming in the spring.
Only a daring villain would dare to break the seal upon the tree. I'm thinking that the tree itself is very old, probably existing as far back as the dawn of time.
The Sealed Angel
An angel is sealed within the tree, as a result of other angels catching wind of her plan; to reset the world. Her lover, a male angel who agreed with her plan, seeks to break the seal.
Strangely, despite his agreement with her plan, he doesn't care whether or not his lover will actually reset the world; all he wants is to be with her once again.