Just something to think about, and I'm not sure if this is out of scope for what is otherwise a very clean, simple mod, but in keeping with the utility belt vibe, it'd be nice if there was an API so client developers could attach "pouches" to the belt.

These could be used to store custom items like Thaumcraft spell foci (there's one in TC6, but it takes up the baubles belt slot entirely), or money (for something like EnderPay).

Looking forward (as you have been) to a more open-api-ish modding future, it'd be nice to allow a natural feeling mechanism like this rather than have every mod fighting for slots.

I can see there are pros and cons - one argument is to have prospective "pouch" developers code directly against curios and use a separate slot. But I can see this leading to a proliferation of so many curios slots in decent sized modpacks that things become truly bewildering.