There is some concept art that shows the Zeffo on some of the pillars on the temple on arcana (where this poem is found) and I would speculate that the returning is of Kujet, whose tomb is now on Dathomir, and whom the great mothers are going to resurrect.
There was an abandoned script by Colin Trevorrow for episode IX, which was to be called Duel of the Fates, I think this was the link from which I read it ( https://www.dropbox.com/scl/fi/m6x97a1d55huciezj6jl8/DOTF.pdf?rlkey=p5v5wjyhgm7vwqlfy2kepgpa3&e=1) and it actually (spoiler alert, and actually, I would recommend you read it, it took me 2-3 hours if I recall correctly and it was a fun read, not that I liked everything but I generally quite liked it (not of course comparing it to GL star wars but as I said it had some cool ideas)) so yeah it actually mentions Mortis. Now seeing how it would have been very important to get Mortis right (due to the meaning it has to everything), and how Dave was actually on the sets of both the Force Awakens and the Last Jedi (KK arranged for him to be there so he would learn the live action stuff and finally move to it, he wasn't involved with the story much as far as I know), it is very likely he was advised when it came to this story. And that and a few more things make me think we will be seeing some elements from it in ahsoka (since they were (if I am correct) actually Dave's ideas).
As far as I remember Kylo was either instructed to go or drawn to Mortis, where he was seeking a great power (just like Baylan is) and when finally some box where it was supposed to be was opened it was empty or sth like that. Now I would hate if this actually happened, fans speculating so much and thinking what he is after only for him to not find anything there, but the idea of both Kylo and Rey and equivalently Baylan and Ahsoka going after what the Father is pointing towards is telling me there could be some overlap. And now for the bigger and the overlap for which we could extract more, the big baddy from this movie was supposed to be an ancient dark side user named Tor Valum, and just look at this concept art of him and tell me he is not a Zeffo! (I know they don't have 6 eyes but the head shape is exactly the same). And so what I am extracting from this is that Tor Vallum for Ahsoka season 2 will actually be Kujet who is going to be resurrected by the Great Mothers. It could just be randomly that the head is of the same shape tho, but I think my theory makes some sense.