No pitches at all would render it extremely difficult for those without perfect pitch, but supplying a reference tone or scale (like Earmaster) and highlighting the appropriate key rather than providing each tone on demand would be a good challenge, I think.

> There should be a hard mode where there are no pitches generated while choosing notes.

I challenge you to the "bird_tweet" instrument in "hard" mode, lol. The bird tweet notes are almost indistinguishable it might as well be silent. It's hard even for me!