This idea sounds alot crazy.
Just wait until I go down to a shop, make payments for a goods, and undo the transaction on my way back home.
And besides, this idea will give miners more work to do than they already have today. So, let's just stick to the rules.
~snip
Jokes aside, people already can "undo" or "cancel" their unconfirmed transaction thanks to existence of RBF and full-RBF. They just need to use wallet which support such feature, such as Electrum.
RBF is a perfect alternative for the OP's idea, but my respect to those who uses this feature frequently.
I actually tried this feature twice, and I was like : "Okay, I'm done".
Paying double fees on two different occasions actually hurts. My patience will do more good than harm.