Adding Async USB is almost a must in this day and age - everyone is running out and getting an Async DAC so this will definitely be a welcome feature.
In the process of adding Async USB, might I recommend that an I2S input be added as well?
That seems like a simple thing to add to the SP3, and would be useful in future proofing the SP3 since this year we are seeing a lot of music servers coming out that have this interface.