Having some dependency or being able to apply a single change in a theme and apply it to that theme or a number of themes at once.
At the moment, if you have a theme per song (so you can have different backgrounds) makes it quite a job when the displays or projector arrangement changes for for an event or in a once off venue, as you need to change every single song theme individually.
Having levels of dependence or a hierarchy in the theme structure may also be useful for other visual aspects, so you can push down a change through all dependent themes. For example:
Output area location ScalingFonts StyleFont SizeAlignmentTransitionsColorsBackground color, image or video
Independent themes per song makes changing the font size or layout really quick and consistent across slides.
The alternative is to be able to have a theme editor capability where you can change the font and size, check a box next to it and then say "Apply to the following themes". Then select as many or as few themes as you want to adopt that specific change
ASIDE: Having an overview Theme view, and the ability to start or go through the Theme wizard would be useful. The Theme wizard is very helpful for creating new Themes. For existing themes it would be useful to have a single view and preview. That way you could edit a single feature in a theme, see the change and then applying it without having to proceed through the whole wizard.