So, in the time that this has been dormant, the seventh chapter of Grand Order has been released, introducing several new characters:
As for things these all lead me to think…
It’s specifically noted that Stheno and Euryale have no real combat strength except what the Servant container has, so young Medusa is probably similarly weak, which makes me think of all three being summoned like that, but as one Servant, similar to the Hundred Faced Hassan, or Anne Bonney and Mary Read.
Tiamat as Beast could serve as another antagonist. She’s both supposed to be the mother of everything, and to destroy humanity as Beast, so maybe Shirou’s high CHA leads her to an Adam and Eve scenario, where she wipes out humanity and then they repopulate the world together.
It’s now possible to have a foursome with Medusa filling 3 of the slots. Also, she and her sisters can now fill 5 classes in a Grail War.
And I can see Rin’s goof-up, the presence of some artifact from Babylon left behind by Gilgamesh, and her/Shirou’s luck botching up her summoning, so while she does get Mordred, she also winds up with Ishtar using her body as a timeshare. Rin most of the time and during daily life, but when something threatens her (or just when Ishtar feels the need), she snaps into goddess mode. Possibly including Ishtar having sex with Shirou before Rin, then letting her wake up to the aftermath.