The ultimate mega man story. This is just my thoughts. It would take place throughout the ENTIRE world. It would be 16 bit graphics (hey mega man 9 is 8 bit). And there would be dozens of bosses. Mayb even a return of the much hated Sigma. Maybe even have a robotic version of Wily and a return of Dr. Doppler and a few other villians.
I think what would happen is all the bad guys, sigma, wily etc. would declare War on the planet and it would be up to X and Zero to save the day.
Dr. Doppler technically wasn't a villain. He was possessed by Sigma. In X3, it's revealed he's a virus. He seeped into Doppler and used his smartz to restore him. Dr. Doppler was freed of his control when X defeated him in battle.
So Doppler returning as a villain wouldn't make much sense. Sigma merely ressurecting him just to possess his smarts doesn't make much sense when his creations were capable of being destroyed by X and Zero.
I've heard that Capcom is already working on Mega Man X9 as a download only title. I heard that it's gonna be using SNES style (aka 2D, 16 bit sprites) graphics.
Anyway, Dr. Doppler coming back would make as much sense as Vile coming back in X2, X3, and then out of nowhere in X8.
Capcom could make something up like uh...Dr.Doppler's memory chips were not damaged. An evil Maverick came and rebuilt Dr. Doppler and improved his design...The evil maverick would be Lumines. You play as X and Zero. Axl makes cameo appearances. You can switch between X and Zero whenever you want. Zero has the saber and the Z Buster again. X's upgrade capsules could have Dr. Cain instead of Dr. Light.
Vile would be a mini boss before defeating Dr. Doppler and then Lumines.
I mean I would say that Sigma is dead after X8, unless Capcom decides to make something up again about him and somehow making him come back to life.