The illusion system restricts items to be of the same type.
This limitation is roughly understandable, as a spear and a sword will behave differently.
Nonetheless, there are many weapons which should have the ability to look like another type:
I was a bit sick of the usual look of my Whirlwind Blades, so I made a very simple mod which replaces their model with the Katana model.
They fit perfectly well!
Same when attacking, there is absolutely no issue with using this model.
This works with many variations, you can even use a pickaxe or cleaver’s model instead of an axe or sword 
So yeah, just a suggestion because it would be really nice and it doesn’t seem like massive work to implement 
The hardest part is to test which weapons (and tools) work as illusions for others, and things aren’t always interchangeable:
→ Using a Greatsword model with a Katana moveset works well, but the other way around doesn’t