Just recently, Yuji Naka and Naoto Ohshima spoke with the Japanese magazine Famitsu, and commented on the origins of the Mario & Sonic series, as well as Sonic joining Super Smash Bros. Brawl. But Yuji Naka also said that he wishes to give another presentation to Sonic Team’s Takashi Iizuka about a potential Mario & Sonic action game. See the quote below.
“Actually, I wanted Mario [and Sonic] to appear together, so one time I went to give Miyamoto-san a presentation on how I want to create an action game which is co-starred by Mario and Sonic. It was in the GameCube days, and I took around four planners with me. It is a pity that the discussion was ‘clearly it isn’t necessary for Mario and Sonic to co-star in an action [game]’ and it didn’t become a reality, but SEGA was able to get the license for the Olympics just at that time, so in order to make the best use of the license, companion characters appeared together. A little later Sonic appeared in Super Smash Bros. Brawl. However, this too came into reality because I requested from Sakurai-san, ‘I want Sonic to appear [in Smash Bros.]’. The Olympics is a bit different from my wish, but I think it’s a good thing that children around the world are happy about them co-starring. But someday I want to realize an action [game] together with Mario. I think I’m going to give another presentation so that Takashi [Iizuka] finds the necessity [of a Mario & Sonic action game] (laughs).
Naoto Ohshima also added:
“I also agree. Previously they were considered rivals, but when I saw them co-starring, I honestly thought that Nintendo acknowledged Sonic.”
It should be noted that neither are at Sega any longer, and haven’t been in many years. Naoto Ohshima left in 1999 to form Artoon and later made a successor company where he still is today called Arzest (who just recently worked on Mario & Sonic tat he Rio 2016 Olympic Games for 3DS). Yuji Naka left a whole decade ago in 2006 to form Prope, where he still is today.
It’s up to Sega (and Nintendo) on whether or not this project ever gets made. Who knows what the future holds!