The developers are challenging themselves to faithfully recreate the 8-bit experience, including limiting number of enemies on screen and making 8-bit music.
In an interview with Mega Man 9 producer Hironobu Takeshita, it was revealed that the team behind the game has included options to enable screen flicker and slowdown to truly evoke the classic feeling of the title.
Players looking forward to Mega Man 9 can expect a fully classic experience in this brand new game due to some self-imposed restrictions in the development of the game. The title, like the classic NES titles, won't include more than three enemies on the screen at once, and when the screen is full of enemy sprites and projectiles the classic screen flickering and slowdown will kick in if you have enable the option.
The interview also revealed that the game has been redone visually at least once because the presentation was simply too complex for what the NES/Famicom could have done. According to Takeshita, developing the music was also a challenge because the restriction to 8-bit was not done by any sort of hardware: it was entirely self-imposed. Though the game is entirely 8-bit in presentation, it isn't being made as an NES ROM and would not be transferrable to a cartridge that would run on any sort of Famicom or NES hardware.
Finally, Takeshita mentioned that with Mega Man 9 they are making it as if it is "the new Mega Man 3, because we wanted to surpass what we did in Mega Man 2." After putting out Mega Man 9 Capcom will be evaluating fan reaction to "give [them] an idea of where to go from here."