"if you would draw"

[folding_music]

eg. Archmage Ascension

If there was a card that said "If you would draw THIS card, you may do X instead", would that mess anything up? what if there was a card that said "If you would draw this card, you may instead pay 1 life, exile it and the next two cards from the top of your library, then draw a card" and then you cast Brainstorm, and drew that card as the first of three?

[Mookie]

Looking at the rules, I see two relevant subsections under rule 614:

614.4. Replacement effects must exist before the appropriate event occurs—they can't "go back in time" and change something that's already happened. Spells or abilities that generate these effects are often cast or activated in response to whatever would produce the event and thus resolve before that event would occur.
Example: A player can activate an ability to regenerate a creature in response to a spell that would destroy it. Once the spell resolves, though, it's too late to regenerate the creature.

614.11. Some effects replace card draws. These effects are applied even if no cards could be drawn because there are no cards in the affected player's library.

614.11a If an effect replaces a draw within a sequence of card draws, all actions required by the replacement are completed, if possible, before resuming the sequence.

614.11b If an effect would have a player both draw a card and perform an additional action on that card, and the draw is replaced, the additional action is not performed on any cards that are drawn as a result of that replacement effect.

As a result of 614.4, I don't believe the suggested replacement effect would actually happen - you presumably won't know that the card you are drawing has an effect until after you have drawn it, at which point in time it is too late to replace the draw.

614.11a implies that if the hypothetical replacement were to occur, then yes, you would apply the replacement to the first card drawn off Brainstorm, then continue drawing the two additional cards.

....that said, if you're trying to design a custom card with this mechanic, I would recommend just doing what miracles do and use a triggered ability.