In RadicalSoda's video on Sonic Mania & Sonic Forces, at 2:02 he shows a screenshot of an anime-inspired fighting game which included a 3D render of Green Hill Zone as a stage.
From the screenshot I can see the game has a 2v2 fighting system, a blue special gauge, and some sort of basketball hoop.
The only fighting games I can think of with Green Hill Zone would be Sonic the Fighters, Super Smash Bros., or M.U.G.E.N., but the screenshot doesn't resemble any of those titles.
What fighting game is this which has Green Hill Zone as a playable stage?
Best Answer
By googling what fighting game features Kirito (the character on the left, from Sword Art Online), I came across the Wikipedia page for Dengeki Bunko: Fighting Climax.
Searching that game brought me to multiple pages featuring the same image you posted, such as this one, which has the following caption under the image:
Additionally, on the Green Hill Zone wiki page, under the list of appearances, this game is listed: