The rules don't need to be revisioned for a card like this, thanks to this little Golden Rule.
"101.1. Whenever a card’s text directly contradicts these rules, the card takes precedence. The card
overrides only the rule that applies to that specific situation. The only exception is that a player can
concede the game at any time (see rule 104.3a)."
The part of that rule there that says "The card overrides only the rule that applies to that specific situation" I think limits Ral Zarek to ONLY override the "Only activate these abilities anytime you can play a sorcery" rule for Planeswalkers, but not necessarily the "once per turn" rule.
Though, they seem to have mashed those two "rules" into one single rule..
But, if I were to see this Planeswalker card, and saw that -1 ability, I wouldn't automatically think "Oh, I can play it as an instant, so that means I can also play it however many times I want!", I'd just think "Ok, I can play it as an instant, so if my opponent throws multiple things at me or a planeswalker I control, I need to choose wisely what it is I want to counter." After all, I'd still be thinking I could activate an ability of a planeswalker only once. Nothing's really given me reason NOT to.
I really just want to avoid making the text so freakin small. It's bad enough as it is.
@The Captivating Vampire comment: Wow, didn't notice that.. It does look similar, though not exactly the same. But still, an outlined colorless Dragon tattoo on the left arm..
@Niv-Mizzet possibility comment: Well, maybe him being known as Niv-Mizzet isn't necessary for what they have in store? And besides, if you were to go into hiding and changed your appearance, would you keep the same name?
I just wanted to add a little something to the comments about Ral possibly being Niv-Mizzet (I know it's all just speculation)
If wizards decides to keep up this double sided card deal (perhaps at least just for the block), and either the next set does in fact revisit Ravnica, or simply Ral Zarek becomes a real card in the next set regardless of the plane, and he IS Niv, then perhaps we'll see Ral Zarek, Planeswalker, who flips over, revealing his true form to become Niv-Mizzet, Planeswalker
I just wanted to add a little something to the comments about Ral possibly being Niv-Mizzet (I know it's all just speculation)
If wizards decides to keep up this double sided card deal (perhaps at least just for the block), and either the next set does in fact revisit Ravnica, or simply Ral Zarek becomes a real card in the next set regardless of the plane, and he IS Niv, then perhaps we'll see Ral Zarek, Planeswalker, who flips over, revealing his true form to become Niv-Mizzet, Planeswalker