Got more ideas as I started sketching, but the general premise is a universal 60% case that adds a BT module up top (the extra space above number row would house the BT tech). Use this as a way to turn your 60% into a sweet retro looking device that can connect to your phone or tablet when needed.
Not only does it look awesome, but it's functional. You could plug the existing USB from the board into the Bluetooth controller, hide that in the top of the case, and then have a connector coming out the back of the case. Wired bluetooth similar to the Matias and Uniqey boards. This would hopefully be fully compatible for any 60% you drop into it (V60, Pok3r, GH60, etc).
I would want a button on the case that turns on the bluetooth and automatically switches to that device. Keeping it on the case would ensure that any programming on the board you put in it would still be supported. Maybe a sweet rocker switch on the right-hand side like a Commodore 64!
I went with the Commodore 64 theme because this is my dream board. I envision it being thick beige plastic with some nicely rounded edges, but of course it could have a more neutral look with black or white keys and more squared off anodized aluminum case.
Dual led red/blue for the top right. Red would be power and it could turn blue when Bluetooth is enabled.
Would need some SA retro keycaps for this, of course