Anyways, a now removed BSIG entry that was promptly captured by WMPU reveals that Windows Phone 8.1 GDR2 or Windows Phone 8.1 update 2 will bring Bluetooth HID support. It adds MAP, HID 1.1, HOGP and Absolute volume control.
We had reported via a tip that GDR2 will bring Settings sorting, Mobile data toggle & Cortana for more. Now Mobile Data toggle and Cortana for more are already here with last GDR1 version 8.10.14219.341, that is rolling out with Lumia Denim update too. So, settings sorting and other changes can still come with Windows Phone 8.1 update 2.
We caught a much more advanced Windows Phone 8.1 version at WP Bench. The version number in question is 8.15.12375.0, which marks a big jump in the version number over 8.10.14219.341, the latest GDR1 version. This we think can belong to GDR2.