I would like this if it could be done for individual nation strings, or even the last mission in the chain. I have a glitched San d'Oria mission that renders it unable to be completed. If I could reset to start over I could clear it, rather than waiting 2+ months for the dev team to even look at my character data.
The downside I see to this is the amount of gil that clearing the missions are worth, and that doing missions unlocks so many zones. You might unintentionally lock yourself out of something.