Approaching this musically, not at plugin level, others have done that...
There are a few things worth trying that might help you solve this without too much effort.

Based on what you've said, you're happy with the melody. Are you happy with the the register it is in?

1. Try it down an octave and also up an octave. If either of those alternatives works for you better, it becomes easier to pick an instrument because it will likely be in the tenor/baritone range or the alto/soprano range. Only certain instruments fit those ranges naturally, so it becomes easier to narrow the instrument choice down.
2. If however, you liked the melody where it is, have you tried changing the entire key of the tune? People sometimes say it is still the same song and that is true with melody and the harmony, but different keys provide different tonality. Try taking the entire tune either down a minor third or up a minor third as one example. You will notice the difference and that may inspire you as well.

That's how I would do it as a starting point, if I did not immediately hear an instrument playing the melody I hear in my head i.e., being to change the octave or the key. Lower melodies can provide a nice warmth and higher more cutting, and some keys may fit better than the one you started with. Trying these might tell you what the right instrument for the melody may be.

Last - knowing whether it is a plucked/picked/hit - or - breath/bowed based melody helps too.

 

Last edited: Apr 23, 2025 at 10:44 PM

Write on piano or acoustic guitar, or a really basic synth patch. If the melodies aren't sounding good there you don't have anything.

 

This happens to me very often. And the solution, of course, is



Because

 

Create the sound that you hear in your head using the synth/synths that you know well to compliment the track. Using eq to create distinct space for each layer is also a thing.

 

And panning to different locations in the stereo field after gain staging is also helpful in giving each instrument its own place.