It would be extremely useful to be able to share remarkup snippets across Phriction documents. A typical usage scenario is dates: you might need to display a milestone in multiple wiki docs but it requires a lot a maintenance to keep all the places in sync.
It could also be used for anything else like a list of items.
It can be done today using Paste but it's quite a bit of overhead and paste is really designed for code i.e. it doesn't do remarkup (right?). Also Pastes cannot be edited as easily as wiki docs because of their specific access control - only the creator can edit them.
A possibility would be allowing a wiki doc to include another wiki doc using a special #include syntax.