This may be a spoiler if it ends up being true, but for now, it's just a theory
Well firstly I think that the humans of Arcadia will discover the aliens. Due to the sneak peak revealed by DreamWorks last year on 3 below part 2, ( m.youtube.com/watch?v=dza61onWtPg ), we know that Eli and Toby find out that Aja and Krell are aliens. But also if u were to refer back to the first episode of 3 below, you can see Aja telling the story of their adventures to what look like humans, possibly meaning that the secret of the aliens are out?
Secondly, I think that Aja and Krell or just Aja will NOT return to Akiridion 5. This is because going to the same scene in ep 1 of 3 below, we see Aja telling a story to humans on Earth. Now unless it's only me, she wouldn't tell a story unless it is finished, and it seems to have finished on Earth. It would also explain the Steve & Aja problem. KRELL may go back home as he is nowhere to be seen, though idk. Plus, it makes sense for at least one alien to stay behind as the description for wizards clearly states that wizards, trolls and ALIENS will fight in Arcadia for one last battle. The key word is alien, meaning that at the end of 3 below at least an alien would have to stay for this description to fit.
This is the description of Wizards: The trolls, aliens and wizards living in Arcadia face off in an apocalyptic battle for control of their magical world. Created by Guillermo del Toro.
This is my theory and if you have any ideas feel free to post below... And do ignore any spelling or auto correct mistakes!