I see a lot of homebrew creations that are close to what I want, but there's like one or two problems with them that means that if I want something closer to what I want, then I have to create a whole new homebrew. If we could just create copies of public homebrews and edit those to have the features we want, then it would avoid a lot of tedium.

I'm pretty sure you can do this. When you see a brew you like, hit the "Add to Collection" button at the bottom. When you do this, does the brew not appear in the drop-down menu when you create a new one?

It doesn't seem to work that way.  I believe you're saying when we access "create homebrew whatever" that "whatever" will be available in the creation drop down as a template including items in our homebrew collections.  I don't collect a lot of homebrew; but I did have a monster from a James Haeck article and a background from a community member.  I tried to create a homebrew monster and background using the aforementioned creations as templates and they weren't available in the drop downs.

It would be _really_ cool if this could be done given the largely inspired but not fully developed idea mine community homebrew tends to be.  I'm assuming there's some technical reason why homebrew can't populate the homebrew creators template dropdown menu despite homebrew integration into the character creator and encounter builder (to my surprise on the latter, that's cool).  I would say the homebrew tools would get more work from me and apparently others if the OPs feature request existed.