Hopefully this thread will spark a discussion of ideas that will most improve the customisability of xenForo.
For me, I think the single most powerful addition to make would be 'custom thread fields'. If this were done in such a way as to allow the definition of content types, to extend 'threads' with additional fields then we could specify which forums use which content types.... and which templates a content type should use. This will open up a huge realm of possible customisations to the forum owner.
What ideas do you have?