That is true, we're sure open to adding more useful bindings to LWJGL 3. Example libraries would be libffi (already implemented), physics libraries, video decoding libraries (libVLC/ffmpeg), etc. The plan is to make the build system modular, so that developers will be able to build and ship only the bindings used in their application. Adding support for a library only useful to a small portion of LWJGL users, will not be an issue.
With that said, there's no point in duplicating effort. If the libgdx bindings can be used standalone, without any dependencies to the libgdx framework, it's a good reason to not go through the effort of adding Bullet support to LWJGL. Another problem is that Bullet is a C++ library and that means the LWJGL3 bindings generator cannot be used (at the moment at least).