Although this doesn't need to be fully Quartz, Luna already has a(n enemy) cast bar, so I reckon a swing timer could be useful as well.
Especially enemy swing timers seem to be few and far between (the only proper one available seems to be the ugly Abar), integrating this into the target unit frame similar to the cast bar would be incredibly useful for tanks and healers.

Turns out there was already an open issue for this: #453.