Allow identifier assignments to reference other identifiers. For example, if a NetID style identifier were assigned (eg: abc12) it should be possible to turn that into an ePPN style identifier (email@example.com) for the same user.
A potential issue here is that such an identifier assignment must run after any identifier assignment that it is dependent upon.
Another potential issue is what happens if the derived identifier conflicts with an existing identifier for a different user. Perhaps generate a notification and don't assign?