Perhaps I'll put some of mine
- Kogasa was Byakuren's old umbrella.
She became a Tsukumogami after Byakuren got sealed. So she wasn't forgotten, it's just that no one was able to recover her back then. Usually it took only a hundred years for an object to transform into a Tsukumogami, but with how Kokoro just came into form prior to the events of HM, I guess that can change.
Probably one of my favorites, and it's certainly rather viable, right?
- Shinki is the "will" residing in Byakuren's Sorcerer Sutra Scroll
- I do think that Maribel might be Yukari.
But, I like to think that when that time comes, Renko will be "gone" and searching for her is one of Maribel's motive to become Yukari. Waiting for her in Gensokyo, the land that they always dream to go to.
- Though she's a bit haughty and arrogant in front of others, Tenshi spends most of her time crying in her room while she's in Heaven before the events of SWR, because of how she's seen/treated by her fellow Celestials.
- Most of the "stray" youkai like the Prismrivers and Tsukumo Sisters, Kokoro and such often came and are under the care of the Myouren Temple.
Sometimes, they participate in holding some events by the temple. I base this idea from that scene where the Tsukumo Sisters was performing an instrumental show and the Prismrivers participating in that
Shoujouji' Tanuki Band story.
And Byakuren is secretly a rockstar.