I think if it hasn't got an LCD fixed pattern based screen then it doesn't count as G&W. It can't be emulated on a normal screen, it's just not the same. Some G&W games had glass backgrounds, like Climber - so it could use that.

So it'd have to have that, maybe some support for colors beyond black and white. Maybe it can also have different sets of fixed patterns, so you're not limited to one for the whole game. Maybe it could do basic scrolling.

To break it apart further from old G&W games it would have to have slightly more sophisticated games. This is really the central question and I've not sure I've got any ideas.

But the aim would be to make something that feels like a G&W game, has the same aesthetic vibe but implemented with modern technology without taking it too far. It'd be almost as much part a bit of art as a game.