I also want to see these sorts of things to at least imagine how others might build on my work for their own uses.
Doing annotations is one of the extensions I definitely imagine having available. I would want users to be able to write some commentary, have it apply to any book, chapter, verse, or run of chapters/verses. (So perhaps one commentary might apply to John 3:16 and 1 John 4:7-8. But I haven’t spent any time yet (if ever!) trying to figure out how to do it.
I haven’t used it for any solutions myself but I wonder if namespaces (only documentation I could find) are a good solution?
You would just reverse the typical use of namespaces: Have the book/chapter/verse first, followed by the book they are from. So something like John 3:16/King James Bible. This would hardcode it in the title of course, which you may want to avoid.
It will also not be as sightly.
The books and chapters are the same (I think), so you would only need to use the name space as verse. That way, you would get a nice link to an all-verse view, but a specific link to KJV if you so wanted.
Practical example using the URL parameter: I open #John 3.16 (or however you lay out the URL), and then drill down into #John 3.16_King James Bible (which is tiddler John 3:16/King James Bible).