To start with I like the approach your taking. If your boards was to have the capabilities of the https://beagleboard.org/x15/ with very decent dsp and I/O pins but use the processor of rockchips rk3399 would give it enough power along and 2gb and 4 gb boards a very useful and extended life to build on for years to come with i/o capabilities to do anything for anything.
Best Regards,
Kevin