At this point we are pretty far into the expansion. Transmog was a great feature that got added. However, the rules in place are a relic of the time it was made. Since then we’ve seen the WoW dev team loosen transmog rules. I believe the same should happen here.
For example, I love the way daggers look. However on my barb I’m running mighty weapons which cannot be transmogged into daggers. Being able to do this is one of those small quality of life things that goes a long way to allow a person to make the character they want to play. And I think that is ultimately what’s important, and its what the point of the transmog system in the first place. Things don’t need to get opened to the point of transmogging a hammer into a cross bow, but I think they should be more open in some aspects such as melee weapon --> melee weapon being allowed.
Edit: Another example I think would be cool is to be able to visually change the elemental damage effect on weapons, that way I could have my swords glow blue or red depending on what I want.