BUR #21547 has been approved by @evazion.
mass update ~utsumi_sakura ~kazakami_saaya ~kogure_ruka ~kuromine_chiharu ~pool:20780 -> copy:sakura_no_sekai -original
The series tsuruse created with the four characters above is called 桜の世界 (Sakura no Sekai, or Sakura's World). Not only does an OR search for the four main characters yield a count of 742 (and there may be a few I didn't tag), several of the comics that were uploaded here with these characters are also included in two published compilations. Because of these facts, I believe this series is a good candidate for getting its own copyright tag.
The only thing I'm not 100% sure about is if every single post with those characters should get the copyright tag. The character's designs and backgrounds seem to have changed slightly over time, as if tsuruse was still figuring them out. Considering that Utsumi Sakura worked as an assassin in older posts but doesn't seem to in the comics, those older posts could probably be thought of as a separate continuity. This may be splitting hairs, but I thought it was worth mentioning in case it was a concern (I've seen something similar with indie virtual YouTubers, for example).
Edit: I forgot to mention a couple things.
As far as I could tell, tsuruse has not drawn any crossover art with these characters. Also, none of the characters listed in the BUR have been tagged on any other artist's posts. Therefore, passing this BUR currently has no risk of causing mistags. If any fanart with original characters is found and tagged before the BUR passes, it shouldn't be difficult to readd original.
There are two other Sakura no Sekai pools (pool #18067 and pool #20211), but all of the posts in both pools have at least one of the listed characters, so including them in the BUR would be redundant. The pool I did include has a few posts without any of those characters but is still a part of the same story.
Updated