Michael McWhertor and Wes Fenlon, at Polygon:
The game would have stayed true to core Mega Man X gameplay concepts, re-imagining his X-Buster arm cannon, his dash and his ability to appropriate the special powers of his fallen enemies. Platforming elements, including X's wall jump, and classic Mega Man X characters would have been re-imagined in new ways.
The videos are quite interesting. They show a Mega Man game that has a completely different style than previous editions. That difference, in fact, might have been the downfall for the game (which was never made beyond prototyping). The article mentions the transition that Metroid made from 2D to 3D, but it's worth noting that the style change was not quite as drastic since Metroid was always somewhat 'dark'. Mega Man, by comparison, has historically been a lot more 'upbeat' in terms of art style.