I think this would be a big feature, but it would be very beneficial. I think we would have to look into the shield model, the texture, the rendering method, and the banner pattern textures.

In 1.20.5/6 there was an update to banner patterns for shield, it seems it is easier to add custom ones. Will look into it over the summer.

The custom shape has been successful, though it is very manual so I don't know how much the library can help with that. A bigger obstacle is the banner designs, as for every custom shape of shield, a new set would have to be created and somehow be used instead of the default vanilla. Simply overwriting the vanilla textures doesn't work too well, even though that is the only way I can find of doing so. Overwriting the textures breaks the vanilla shield's pattern and it limits the mod creator to only 1 custom-shaped shield.

Additionally, the custom looks weird in the hand opposite of which it was designed for. For example, I designed this test shape when the shield was in the main hand, so it looks weird in the offhand.

We would need to write our own banner rendering system that can take in custom sets of banner textures and render them on the shield

for every new shield shape the end user would need to add a new set of banner textures to match

Yeah that was the biggest issue I faced. I couldn't find how to direct rendering for the custom shield to a custom set of banner patterns. I had to replace the vanilla textures. I think I attempted a mixin to change the path, but I'll have to look at it again.