In short can it be done? Say if I wanted to go online with my iPad Air using tethering on my S4 but for whatever reason it had to be connected by a cable instead of the iPad's wifi, would that be possible? I'm presuming not, seeing how difficult it appears to be to find a lightening port to micro USB cable.

For the absence of doubt it would not be for any purpose considered 'illegal' etc

It's not illegal, it just won't work. You can't attach and iPhone to an iPad, or an Android phone to an Android tablet by a cable and allow the tablet to use the data connection. Years ago you could attach a phone to a PC via a cable, but nobody does that now. It's done by Bluetooth or wifi.

No I know it's not illegal or anything but that was just in case the mods though I was up to anything etc

I guessed as much, I just get a little over-paranoid with Bluetooth and wifi sometimes. Clearly I'll just have to suck it up in future

Cheers though.

https://m.wikipedia.org/wiki/USB_host

Think it will catch on?
But yes, USB tethering is not really used anymore, being of limited point from the start.