So like... Scathe is just a really bad spell, right? You almost never want to touch Scathe outside of extremely niche circumstances. Now, I'm not advocating to remove Scathe or buff it so much it becomes part of BLM rotation and heavily alters it.
However, it could be given a minor buff so it becomes a viable learning/emergency tool:
Additional Effect: Extends Astral Fire or Umbral Ice duration by 6s to a maximum of 15s
By extending AF/UI a bit, it allows Scathe to be a decent tool to recover for forced movement while progging or learning the job. It also works if learning BLM forget to refresh AF/UI and there's not enough time to do so with a regular GCD. It's still far more effective and less of a loss to preplan movement and cooldown usage, but it could save BLM from dropping their AF/UI completely and having to restart their rotation.
Then again, it might just promote bad BLM who run around spamming Scathe at the drop of a hat to maintain their AF/UI...