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.
I guess this is only applicable when the transaction has gotten 0 confirmations. This too should have a big risk of getting a stuck transaction usually when there is no transaction with higher fee to replace it and what if the the transaction wasn't marked as replaceable in RBF I guess Reversal will be impossible.
for full-RBF irrespective of been marked as replaceable once there is 0 confirmation i guess it can re replaced with a transaction with higher fee. But what if there are no such transactions for a long time how long will the network congestion last?