You are correct, plugboards cannot change series or a host of other fields. Rationale: Plugboards exist to work around limits in readers that don't use the rest of the metadata, putting data into fields that the readers do use. For readers that are able to use more of the metadata, those fields should already be correct. I am an FBreader user, and that is certainly my experience. I have seen reports that other android readers use calibre's metadata to good advantage, e.g., moon+ IIRC.
I see no advantage in complicating plugboards more than they already are to add changing fields that in the vast majority of cases contain correct data, so I won't make this change. If you wish to make these changes, then make sure that Kovid will accept them before you spend much time on it.