Just my 2c. The whole point of usb dac dongle is to have a small device to enhance the audio quality of your source like a smartphone. The idea of DC Elite is to maximize this enhancement in the smallest allowable footprint. Adding Bluetooth functionality means adding internal battery, power filtering, and other chips, making it bigger and bulkier. And, CB19 gives you an option to use any external power bank. I can guarantee, if iBasso going to come up with a piggyback battery module, there will be a ton of complaints that capacity is not high enough or it creates some interference, etc.
At this point, just keep DC Elite as is, use CB19 for convenience if you want to, and let some 3rd party to come up with a solution