Can you use Mystic Reversal on Windwall and change Windwall's target to Mystic Reversal itself to make Windwall fizzle?

Ruling: No, you cannot target Mystic Reversal with Windwall. Mystic Reversal will have already resolved and left the chain by the time you choose new targets for Windwall.

Sequence:

• Mystic Reversal resolves and takes control of Windwall
• As part of Mystic Reversal's resolution, you may choose new targets for Windwall
• After following all instructions (including choosing new targets), Mystic Reversal goes to the trash
• Mystic Reversal is no longer on the chain when target selection occurs
• You must choose a legal target that exists on the chain (Windwall was already targeting something)

Nuances:

• You could redirect Windwall if there's another effect on the chain (e.g., if you played Discipline, then Mystic Reversal, you could target either the original spell or Discipline)
• The "may choose new targets" wording is important - it's optional
• There is ongoing debate about whether choosing new targets requires re-finalizing the spell and what implications this has for additional costs like deflect, but this doesn't change the core answer