Here's my idea of what a completely hummed language could look like, refined from my post on c/neography a couple weeks ago (here). It only uses 4 or so different sounds, /Ɂ/, /h/, /m/, /ɴ/, /m̰/, & /ɴ̰/, all but two of which can have any combination of 5 pitches to make an arbitrary number of tones.
I won't go into much detail on how exactly the writing system works since it's mostly unchanged from my first post about it. Tones are made by stacking pitches from top to bottom, alternating between the left and right sides. The bottommost pitch is always on the left. Words are read from top to bottom and left to right. A diagonal line can be added above the horizontal bar tones connect to to show that a tone uses /ɴ/ instead of /m/. A bare vertical line lengthens the last pitch of the previous tone and an arch connecting two tones marks something between an affricate and a diphthong, as opposed to having a slight pause (but not a glottal stop) between tones.I don't have a romanization system, i've just been using IPA with Chao numerals and an affricate marker for connected sounds.
I probably won't ever make a full conlang out of this, but other people are welcome to expand on what i've done here. All the examples i gave in the attached picture are just sounds to show off the writing system and don't mean anything.
Honestly very interesting concept