In the Sarah J. Maas universe, time travel is possible. when Aelin saw Prythian and Lunathian, she went forward in time. Despite it being an unpopular opinion, there's a bunch of evidence for this - for example, the Harp singing "we shall open doors and pathways, we shall move through space and EONS together" [eons meaning a 'very long period of time'].

Many thousands of years ago, and prior to the Asteri's invasion of Midgard, there existed another civilisation. Part of this civilisation lived in a place called Parthos. More specifically, when asked what the Crescent City world was before the Asteri's reign, Tharion noted that "ancient humans and their gods dwelled here." An exact description of the Throne of Glass world.

During the series, Aelin ricochets off “the bottom” of her power and describes that going up is so much harder than plummeting down. It reminiscent of Bryce describing the difficulty with the Drop and her own journey. To get Aelin out of this trance, Lorcan spots Rowan anchoring Aelin back to reality… something folks need to successfully complete their Drop.

There might be an empty throne in “a hell-realm” after Aelin had killed Hellas, along with the other gods from TOG?! Not to mention the suggestion that Erilea is almost certainly linked with the same Hel as Midgard, which was essentially confirmed in House of Sky and Breath, where we learnt that the fae-shifters from Midgard likely came from Erilea.