That's what i did for burning embers in ncb:
http://s.cdn.wowinterface.com/preview/pvw56932.png (topmost, bar texture is a bit ugly)
So, I'd say you should go for a medium sized bar that would be good for both demonic fury and burning embers.
You also can utilize it for other classes, for example tracking duration on arcane charges.
Potion idea is good, but as with blizzard's default it might not be always easy to tell whether it's full or 90% full.