Daniel Ekman SA2KNG
Basic problem with the request is that it doesn't contain a suggestion on how the hardware should work.
I'ts certainly not impossible, just that it has no simple solution that doesn't have the risk of impeding on other functionality. There ain't too many "spare" pins available. There will be only one firmware, so all accepted features are implemented and then you change some parameter in the menues that selects the behaviour you want.
For example; If you want one more pin for multiplexing (bank a/b) it will probably come from this list: gps data, gps pps, button 1+2, encoder a+b. If you choose one of them, what protects them from getting toasted if someone configures it wrong ?
Another way of doing it would be to steal the first relay and make that multiplex 5x2 relays, add some 10 additional diodes and wiring.
And a quite different way would be to use an external shift register and repurpose the relay pins to go to one or more of these in cascade. This requires another board to be inserted between the qcu and the relay boards as to decocde/drive these.
The next problem is figuring out which one of these will be best for the future and then sticking to it. It needs to be compatible and safe for current and future hardware.