To answer the questions above, Megaman 9 and 10 could technically be made for the NES as the sprite size and color limitations, four channel audio and even the scanline limits are kept within the limitations of the NES. That being said, both 9 and 10 were programmed in C++ using completely new code as Capcom didn't keep the NES source code since making the complete works ports on PS1 in the late 90s. There was even a sound engine created to emulate the 2A03. While mostly accurate, the sound engine used for those games can produce irregular beats whereas the real 2A03 can't. What that would mean is to get Megaman 9 or 10 on ported to the real NES, the games would have to be completely rewritten from scratch.
It's not impossible, and given some of the incredible homebrew Megaman projects like Megaman Ultra, Rockman no Constancy, Rockman 7 and 8 FC and the upcoming Megaman Odyssey, who knows, it's possible one day we will see unofficial ports of 9 and 10 on the NES as long as Capcom doesn't C&D the whole deal.