I now think that it is too late to take relatively easy swapping away from players, and any new restrictions Blizzard implements will simply enforce not allowing swapping in combat and while a spell is on cooldown.
Rather than encouraging ‘builds’, it seems Blizzard will want you to focus instead on what 6 spells you have active at any one time. Builds have now become ‘dynamic’, and ‘situational’, according to personal preference.
In that case, there is a better way to do this then with the current UI.
Currently the player opens up the spell/rune window, selects a slot and then a spell and rune combination. It isn’t a bad UI – if swapping wasn’t allowed it would be a great UI - it's just that there really isn't the need to open that window. The reason I say that is because I think most people will rune a spell the way they like it, and will only swap SPELLS regularly, NOT spells AND runes regularly. That's not to say swapping runes doesn't have its uses, it’s just that it will be MUCH less frequent then spell swapping, and so the UI doesn’t need to be streamlined for that.
What happens to the current 6 slot spell/rune page?
Well, if all spells become accessible, then there is no need for the page to limit it only to 6 slots. It would probably show ALL spells available to a player instead, and allow for all to be runed.
Disclaimer: Of course there is no swapping in PvP. Perhaps if you want to change your spell combination in PvP you have to sacrifice instant resurrection in the Arena (less time for you to gain points) to enable you to make a new combination (with a timer cap of course).
EDIT: I did suggest an alternative but it has been shown to have a fundamental flaw, so there's no need for that bigger wall of text.