the "pwm" you can do on a NES sounds really bad

i wouldn't rely on that for anything apart from techno space-modem sounds

Just make a simple ping, followed by a sudden drop in volume with a slow decay.

"NES has thin, thick and fat. those are your shapes."

Yes and no.

You can mix the pulse waves together for more dynamic sounds.

" The versatility of this technique is not to be underestimated; it is
          integral to many fuller-sounding NES soundtracks. It enables the
          composer to choose from a wide range of instrument sounds far beyond
          the simple ping of the pulse wave's four duty ratios."

Here's the full quote from this document:  http://nesdev.com/mckc-e.txt
"
Timbre Macro
     @[num] = { - | - }
          Accepting values between 0 ~ 127 for [num] and 0 ~ 15 within the { }
          brackets, this macro will change the duty ratio (timbre) of the sound
          while playing, as follows:

               0... 12.5% (thin, raspy pulse wave)
               1... 25.0% (smooth, thickly timbral pulse wave)
               2... 50.0% (clear, thin bell-like square wave)
               3... 75.0% (identical to 25%, but phase-inverted)

          Multiple values are separated by spaces or commas.

          The first value defines the initial timbre, and the last number the
          final timbre, to be held until the note is stopped. In between,
          however, the timbre will "sweep" through each value, advancing once
          each frame, until it reaches the end. In this manner, you can give
          each note an "attack" that will make it rapidly change from one timbre
          to another when the sound starts.

          The versatility of this technique is not to be underestimated; it is
          integral to many fuller-sounding NES soundtracks. It enables the
          composer to choose from a wide range of instrument sounds far beyond
          the simple ping of the pulse wave's four duty ratios.

I think going for the attack/decay (volumewise) will probably do more to define "piano" sound than messing with pulse widths.

NES has thin, thick and fat. those are your shapes.
you can try modulating between duty cycles reaaaaally fast in lieu of FM synthesis - have a look at some of neil baldwin's work for ideas (i don't know if it's open source though)

Would researching the shape of different instrument wave shapes help, you think?

well you are mostly working with square waves. maybe do a curved volume slope down and then to try to adjust the duty cycle to have a quick jump from 12% to 25% and sustain at 50%. its never going to sound like a piano but it might kinda emulate the sound of the initial chaos when a string is struck on a piano and it smoothing out as time passes.

Hey guys, I'm making a "de-make" and hoping to produce a nice piano sound without samples for the NES. Anyone know of some good settings with MML to produce this kind of timbre?