In light of Bloomsbury announcing 4 more SJM books, let's not forget about SJM's other planned series...
Back in 2015, SJM started a Pinterest board for 2 new book series that were connected in some way; one was Crescent City, and the other was called Twilight of the Gods. On her Twitter, SJM mentioned that she had been working on both for quite a while, and that it was soon time to release them into the world.
Around the same time, a series called 'Twilight of the Gods' was mysteriously added to SJM's official Goodreads catalogue.
'Twilight of the Gods' is another word for Ragnarok; a famed tale of Norse mythology where the Gods and giants/demons across all worlds came together to fight a giant battle that signified the end of the world.
Thus, I believe that 'Twilight of the Gods' is SJM's next series; it will be a Ragnarok retelling, and all of the characters from TOG, ACOTAR and CC will join together for a cataclysmic battle against the Asteri and the Valg (because lets not forget that Orcus and Mantyx are still unaccounted for...).
'Twilight' is also another word for Dusk; considering the lost Dusk Court, and 'Dusk's Truth' - it matches up perfectly.
With this in mind, consider the numerous references to Norse mythology that SJM has already scattered throughout her books:
Feyre as Freya: Freya was perhaps one of the most renowned Norse goddesses, and was Queen.
Nesta, Emerie and Gwyn as the Valkyries: An obvious one, but the Valkyries originated from, and had a huge part to play in Norse Mythology (Ragnarok especially).
Lucien as Loki: According to Norse mythology, Loki is often depicted with long, red hair. He is also seen as a God of fire, and is commonly associated with foxes.
Danika (Fendyr) and Fenrys as Fenrir: Fenrir was a renowned monstrous wolf of Norse mythology. Fenrir being 'unleashed' is one of the key events of Ragnarok.
Hunt as Thor ('Thurr'): During Ragnarok, Thor has a famous battle against the 'Midgard Serpent.' Consider the snake that Hunt is holding on the cover of HOSAB.
Midgard: is the 'Earth' world in Norse mythology.
Hel (spelt the same way): Is the 'underworld' of Norse mythology.
SJM also used to have a Pinterest dedicated to Twilight of the Gods, and although it is deleted now, @oohthefeels on tumblr made a post back in 2016, explaining what was on there:
The cool ass armour in question:
Ultimately, I personally believe that SJM has been planning everything for a long time - and that her plans for the multiverse are so much greater than she's telling us.
And a full crossover multiverse series? I for one, think that would be epic.