What happens if a previously balanced pokemon becomes over-the-top by running Z-sets? This is something I definitely think is going to happen eventually. I think it's agreed that we can't ban a Z-crystal or Z-move when it's only broken on one/few pokemon, but at the same time it would be a shame to lose an OU staple because of this new mechanic, especially ones that keep many other pokemon in check (without being overcentralizing, that is.)
What happens if a move with low distribution is too strong as a Z-move? For example: in the event that Porygon-Z's Z-conversion set is found to be too strong in whatever tier, would P-Z, Conversion, or Z-Conversion be banned?
If a Z-move is inherently too strong, would it be banned, or would the most potent users of it be banned? I think Hyper Beam and Giga Impact are candidates, they hit very hard with no drawback (sans the item and moveslot,) have very good neutral coverage, and are given to virtually every pokemon. However, if they're only broken when used on a few specific sets on, say, a dozen pokemon, and fine on the other hundreds of pokemon, what gets banned?
If a Z-move is banned, would the combination of that move and its Z-crystal be banned, or would using it in battle be banned? Say for whatever reason that Z-Bulk Up is too strong on nearly anything that gets it. Bulk Up still has competitive use, unlike things like Splash or Celebrate, and conceivably one could want to run a set that includes Bulk Up and another Z-fighting move. Would this be accommodated?
That's all I can think of right now. There's certainly gonna be some decisions that have to be made.
I guess that, most of the time, if a z-move ends up being broken on every user or almost every user, we shouldn't ban the users, nor the base move, nor the z-crystal, but rather, we should ban the z-move itself.
And I don't mean doing a complex ban of "z-crystal+move isn't allowed", because, like you said, that would bring some problems. Imagine if Z-Hyper Beam ends up being too good the meta, so Normalium-Z + Hyper Beam is banned. But now, if I want to run Z-Protect and Hyperbeam in the same set, I'm not allowed, even tho I wasn't going to use Z-Hyper Beam.
I say we should ban a certain Z-Move in the same way we banned Mega Rayquaza. With Mega Rayquaza, we didn't ban Dragon Ascent, and we didn't ban Rayquaza. We banned the act of Mega Evolving Rayquaza, the act of pressing the "Mega Evolution" button in the bottom screen.
In this way, in a metagame where Z-Hyper Beam is banned, you can use Normalium-Z AND Hyper Beam in the same moveset, but you can't click the "Z-Hyper Beam" move.
In a simulator, this is easy to implement (I guess).
In 3DS, we just tell the players "You can use Normalium-Z and Hyper Beam in the same mon, but
don't you dare to click Z-Hyper Beam", in the same way we tell players to not click Spore against 2 different Pokémon or to not go nuts with Leppa Berry + Heal Pulse. We just ban the act of pressing the "Z-Hyper Beam" button.
In a scenario were a Pokémon gets broken with a certain Z-Move, without the Z-Move being broken when used by other Pokémon, or without the user being broken by itself (Like, Z-RD Manaphy), I think we should ban the Z-move, even if it is broken only in one Pokémon. So, in case of Z-RD Manaphy, Manaphy doesn't get banned, but rather, Z-RD gets banned, even if Z-RD Keldeo is not broken.