Just let me blow your mind with this
SONIC MAY BECOME PLAYABLE IN MKS []
also Geno[]
I know it sounds stupid, but here is my proof.
Found this tweet while looking around, it shows a obvious fake character select screen. It also showed WwwWario, Dr. Mario and Geno. Two joke characters were there too, Sakurai and Ridley (and Sonic.)
This was on September 1st.
On September 16th, they added Dr. Mario in V.0.5
On October 1st they added WwwWario in V.0.6
This MIGHT mean we will get Geno
And Sonic, because he makes A LITTLE sense, being Mario's rival.
(BTW, the image in the tweet is marked with unsafe content, it is not.)