As people may have noticed, Servant character tags are kind of a huge mess right now. The increasing cross-pollination of Fate works, the reduced relevant of class names relative to true names, the explosion in Servants per class with the advent of FGO, the huge difference in popularity and awareness of FGO vs the original works and Danbooru's own relatively sloppy methodology in creating tags for newly minted characters means the current character tags are less useful than they should be and often get in the way of efficient tagging.
Moreover, Servants are almost entirely public-domain characters, which means their names and tags have to be shared with other properties, and not all tags play nicely. (Not to mention class names) lancer, saber, archer, bedivere and gilgamesh, for example.
I pointed this out back in forum #119888, but nothing happened. Probably because it's a pretty big undertaking.
So I finally worked up the effort to do it myself!
Linked is a spreadsheet of every character tag present in posts tagged only with fate_(series) and fate*/rojiura_satsuki_:_chapter_heroine_sanctuary/koha-ace copyright tags, along with proposals for renaming. (The data was taking using the danbooru BigQuery)
Some items I am unsure about, so I left them with question marks. I'd appreciate commentary. Once it's finalized I can take the spreadsheet and generate a bulk update request for the aliases and mass updates (including cosplay tags). Then I can do some manual work on some problematic but thankfully small-scale tags (such as Jekyll and Hyde sharing tag).
With regards implications:
This also cuts down on the number of character tags some works get. Imagine somebody cosplaying as MHXA or JASL. (For example, post #2627585, Lancer Alter cosplaying as MHXA for a grand total of 5 character tags on a single person)
It's done, for the most part.