The fact that only the Japanese version comes packaged with 6-button controllers has me wondering what that might imply for possible differences in the game line-up. So, I checked out the Sega JP archives site for a full list of 6-button enhanced games that were released in Japan.
https://sega.jp/history/hard/megadrive/software.html
https://sega.jp/history/hard/megadrive/software_l.html
Published by Sega:
Ex-Ranza / Ranger-X
Art of Fighting
Eternal Champions
Virtua Racing
Bare Knuckle III / Streets of Rage III
Greatest Heavyweights
Yu Yu Hakusho Makyo Toitsusen*
Samurai Spirits / Samurai Shodown
Story of Thor / Beyond Oasis
Published by 3rd party licensees:
Street Fighter II' Plus / Street Fighter II' Special Champion Edition
MiG-29 Fighter Pilot
Dyna Brothers 2*
Battle Mania Daiginjo / Trouble Shooter Vintage*
Davis Cup Tennis
Dragon Ball Z Buyu Retsuden*
Mortal Kombat
Fatal Fury 2
Super Street Fighter II: The New Challengers
Mortal Kombat II
Gouketsuji Ichizoku / Power Instinct*
NFL Quarterback Club '95
True Lies
Maximum Carnage
Batman Forever
* Japan exclusive
So, Dyna Brothers 2 is the only announced game on the Japanese system that's on this list. None of the other Japan-exclusive games on this list strike me as an obvious inclusion. Power Instinct is an Atlus game, so it wouldn't be difficult to include it now that Atlus itself lives under Sega's umbrella, but it's not exactly a marquee title for the system. All the other Japanese exclusives here would have some kind of license to worry about before they could be included.
I wouldn't expect games like Eternal Champions, Streets of Rage III, Street Fighter, or Mortal Kombat to show up on the JP list if they weren't also showing up on the US roster as well, and those games are crippled when played on a 3-button pad.
Although I also know that Contra is a 6-button enhanced game, despite that distinction not being noted on the linked website above. I think you switched between free shot mode and fixed shot mode with the X button.
I don't remember at all what the XYZ buttons are used for in Battle Mania II and I know that I've beaten that game in the past.
And although there hasn't been any indication that they've considered including CD or 32X games on this system, I decided to check those pages as well out of curiosity:
https://sega.jp/history/hard/mega-cd/software.html
https://sega.jp/history/hard/mega-cd/software_l.html
https://sega.jp/history/hard/super32x/software.html
https://sega.jp/history/hard/super32x/software_l.html
Sega CD 1st party:
Wing Commander
Sega CD 3rd party:
Might and Magic III*
Shin Megami Tensei*
Mortal Kombat
WWF Mania Tour / WWF Rage in the Cage
Fatal Fury Special
32X 1st party:
Star Wars Arcade
DOOM
Virtua Racing Deluxe
After Burner Complete
Cyber Brawl / Cosmic Carnage
Metal Head
GOLF MAGAZINE PRESENTS 36 GREAT HOLES STARRING FRED COUPLES
Stellar Assault
32X 3rd party:
Mortal Kombat II
NFL Quarterback Club '95
The original Shin Megami Tensei would be an interesting novelty for legacy reasons, and it's another Atlus get, but again, not exactly a significant representative for the system.