I've been kinda out of the Fate loop so what other lore and plot stuff did I miss?
West Side? Nothing much, as Babylon hasn't been released yet and it's still on Camelot currently. East Side? Besides having done 2 of the Lostbelts, nothing much either, besides Sitonai who is a Pseudo Servant Ilya. I honestly can't tell you who if Sitonai is a myth character in terms of mythology or whatnot, as she she seems to be along the lines of Passionlip and Meltryllis where she's a fusion of different goddesses and myths, using Ainu Mythology, Norse Mythology(Freyja) and Finnish Mythology(Louhi).
That said, she's definitely using UBW Ilya as her base, as her Bond 10 Craft Essence talks about Berserker and she clearly remembers what Gilgamesh did to her. And she has a big polar bear named Shirou.
Edit:
@Shishœ
OH. On the other hand, apparently Strange Fake has released another chapter featuring more Alcides(Archer/Avenger) and Hippolyta. Not only that, but Ishtar actually makes an appearance using a Homunculus body. Compared to Rin!Ishtar, where her base is holding back some of her pride and whatnot and is often the butt of jokes, the Ishtar in Strange Fake is absolutely sadistic, non-comedic and is apparently about to drop the adult Bull of Heaven upon Gilgamesh and Enkidu in the middle of a city.