Follow the path of Mega Man 11 for Mega Man 12, but use top notch 16 bit-like pixel art as the one seen in the CPS2 games following the trend of Sonic Mania and Dotemu pixel art games. Regarding the character design of the bosses, use the NES ones as inspiration.
For Mega Man X9 or Rockman & Forte 2, use the SNES games pixel art as inspiration. For Mega Man Zero 5, use GBA games pixel art as reference.