Well, Legends WAS it, except not.
The idea was to have a game that plays like Starfox Assault's on-foot mode, but without ammo to worry about except for the special weapons of bosses that you gained in very much the same way as any other Mega Man game. In fact, it's not even a different series; this is just a side series of Classic.
At any rate, it's 3D free-roaming. There are also stages like, say, Quick Man's stage, where the camera would go top-down, so you could see how you are falling, instead of relying on screen progressions/memorization. The 3D element also eliminates a lot of collision damage (particularly with bosses). That said, you could still have one-block wide platforms and linear levels (Wily stages, for sure) to replicate the same great stage design.
What else. Lock-on feature, sure. Shop akin to MM8's. One of the buttons turns you either 180, or in the direction from which you last got hit.
Then, use this same engine, except stylize it differently for any other series you wanted to use it in. It could've even been the basis for MMX:CM's navigation.