I've seen this effect on various custom card creation communities over the years. Personally, I think I liked the variation best which was an ability on a creature with the mana cost (non-). I think it might have been a variant which said something like "If you would choose a mode, you may choose all of those modes instead."

Anyway, I doubt this needs to cost since it doesn't do anything on its own and you need quite specific cards to pull anything very crazy with it - at least, as far as I know.

I just now noticed that this actually speaks of "choices"... You mean the modes on charms and such, right? Is there anything else it's supposed to affect?

I guess "You may" would count; but "Ok, I don't... alright I will" is all you can do there. I guess a very few of them have both "If you do" and "If you do not" sides which would be usable.

Potentially good with Addle - "Choose a colour" and "Choose a card". Pretty good with any non-random discard, actually.

And that's assuming you cannot use it for things that have targets to get multiple targets. Which would be.. rather good.

Okay, it's certainly mythic worthy then.

What's a good way to have it's ability affect modal spells? This set is pretty heavy on them, and I want it to affect cards like Caretaker of the Core or Echoing Growth.

... doesn't it already? Isn't "choosing one" making a choice? So they kind of transform into "choose one or two".

I think the question is for a wording that explicitly affects modal spells like Funeral Charm, Austere Command and Mystic Confluence.

There the wording reference in Tahazzar's first comment is useful: The "choices" on a modal spell (bullet points) are called "modes", so "make a choice" becomes "choose a mode".

Choosing a mode is a much more concrete concept in the rules than the abstract making a choice.

I would either way suggest "spell or ability" over "spell or ability".