I’ve been curious about adding a filter of some kind to either the input or output to change the tone. I’m mainly interested in achieving a more saxophone-like sound. Humming the same note that you’re playing gets interesting results, though it depends on the ability to match the note and on the player’s vocal range. Humming a drone also works, but takes some concentration.
I’ve tried putting a pitch-pipe type free reed in my mouth and blowing through it into the mouthpiece and that is interesting too. I may fill a mouthpiece (I had a bunch 3-D printed so I have some to spare for experiments) with silicone leaving a small hole to fit a pitch-pipe, that can be removed and swapped to change the drone pitch. The pitch pipe has less of an effect on the melodica reeds’ sound than humming.
Other input-side additions could be interesting… bird calls, particularly crow calls can have a good sound. But messing with the output could tweak the tone without having to add an additional note. Has anyone experimented with adding something semi-permeable, for example very thin latex, to the exit holes of the case, or maybe on the back side of the plate where the air that passes the reeds first comes through? It would require more air pressure to sound the note, and may produce an extra note (which I personally don’t want) but may be interesting. Or maybe thin paper could give it a buzzing quality.
I’d really like to find a way to make the melodica sound more horn-like. Using tonguing to articulate the attacks helps, but it’s more subtle than I’d like. I’d love to sound more like a sax than an accordion or harmonica. Maybe I’m chasing something impossible.