Either a roboport that can exchange bots with the port above/below it or a bot access point to allow bots from a nearby ports to access lower levels.

I'd argue for the latter and say maybe roboports should be restricted from being placed on lower levels.

Edit: This issue may be a little more complex than I previously thought, part of the issue I'm envisioning is about sharing a construction inventory between the surface and sub-levels, maybe the latter option solves this too, but it may not be as elegant from a behavior modding standpoint...

This is quite hard to do, can't promise it will be implemented

What if the bore hole was able to pass materials requests between levels, similar to space station hub behavior?