So, this story started with me having a terrible idea for a card, and instead of changing the card, deciding that the rules should change instead.


raz informed me that the current effect creates a lot of undesirable gameplay because of undo rules for casting spells, which I agree with. However, given that the game already specifies certain constraints on what mana abilities can be undone, is there a reason it shouldn't be extended to effects such as the one above, and Rhystic Cave? Are there any disadvantages to prevent undoing mana abilities that involve an opponent making a decision, or reason it wouldn't work at all?

I'd say your card is fine. It's not hard to reverse, so rule 717 doesn't need an exception for it. Also, you don't want to activate it while casting a spell, because that gives the opponent extra information. So you just activate it, opponent chooses a color, then you go on and cast something with your new mana.

It's very important to note that rule 717 is called "Handling Illegal actions". It's not "Handling legal actions that I want to reverse because I feel like it". It's also not "Handling illegal actions committed on purpose, also known as cheats". It's there to fix honest mistakes, like when you tap three forests and three mountains to cast Force of Nature and then realize that you can't. Dishonest mistakes are fixed by disqualifying players.

Rhystic Cave currently has the restriction "Activate this ability only any time you could cast an instant." It would be better to just use that same restriction instead of changing the rule. There are unfortunately already times when you can't predict in advance if your opponent will make your action illegal (e.g. Evangelize + Icefall Regent and another creature with only available), but the fewer cases of that, the better.