As fans of the Mega Man franchise well know, a Mega Man series with a definitive ending is a rare thing indeed. To date, only two have yielded what might be considered a satisfying conclusion (though that's never stopped fans from asking for more): Mega Man Battle Network and Mega Man Zero, the two Game Boy Advance series which largely defined the Blue Bomber in the early aughts.
Both saw their own respective continuations set long after those conclusions, Battle Network with Star Force and Zero with ZX. It's the latter that seems to have particularly intrigued people, as the world was effectively rebuilt from the brink of destruction into something of a relative utopia -- at least compared to the series between it and Classic. That's left the question of what exactly happened during that time.
At the moment, Capcom is not forthcoming with answers, as Inti Creates' work on the ZX series was halted following Mega Man ZX Advent. As a result, a group of fans under the banner of "Team Einherjar" have lent their own ideas to a new fan game titled "Mega Man ZX Genesis."
Set between the end of Mega Man Zero 4 and the beginning of Mega Man ZX, "Genesis" follows Ciel in her development of the BioMetals and the conflicts she might have faced -- a bit reminiscent of "Project Ciel/RCL," actually.
As for "Mega Man ZX Genesis," Zerolegends says that a playable alpha demo is available now, and provides the following trailer: