This is possibly too complicated, but in actual play, it will almost always be the same as "Exile ~ and return it to the battlefield", ie. "Do all ally things twice".

However, I wanted something that sounded snazzy.

And originally I wanted something that could interact with the planeswalker abilities. Unfortunately, I can't think of any wordings that would quite work. I think you can copy the planeswalker ability if you play an Ally Instant in response to it? But if this is triggered by Gideon, Leader of Heroes' second ability putting an Ally into play, that ability will still be on the stack as this is triggered (?) but will be gone by the time it resolves (?)

Yep, I think your comment is entirely correct. A self-Blinking Ally is definitely cool. Join the Ranks will get you two extra copies of a planeswalker ability with this out.

Two of these on the board... I have a feeling they actually go infinite. That could be a problem.

Hm, you're right. Hold on a mo, let me slap "legendary" on it... no, that's not actually a good solution.

Probably best is to make it ": copy any number of triggered abilities from an Ally source", but that makes a nice card with little connection to the Planeswalker. Unsurprisingly :)

Or, indeed, actually making it blink would be an interesting solution (though combine badly with Kazuul Warlord). Maybe that should have been one of the planeswalker abilities?

Perhaps this should specifically call out Allies? I missed where you were going with this, myself. I assumed there was some interaction with the next card down the line that made Allies have flash or something. If the card read "copy target triggered ability from an ally" it would still do what you intended it to do, and told the people playing at home what they should be doing with it.

Yeah, there's a bigger problem with this than going infinite. Two of these by their lonesome cause a game lock, since they are forced to copy each other's abilities. If I have an archmage, and I play an archmage, and there are no other allies, the game ends in a tie.

It's not a lock, because "any number of" includes zero. Once you have your googol of allies, you just start choosing to copy no abilities.

Is this true? I know that you can no longer divy Fact or Fiction into a pile of 0 and a pile of 5. I can understand how there needs to be a card in a pile to be called a 'pile'. But I'm not 100% sure its not because you can't call "0" anymore when asked for a number.

Found it: 107.1b: "Most of the time, the Magic game uses only positive numbers and zero." so yeah, the no 0 cards in a pile rule must be divy mechanic only.