Okay, I havent watched the video of SDCC yet, but I keep seeing SO MANY POSTS about theories for The Dragon King, three in particular, and I wanna make my own stance on them currently.
Aaravos' Return/The Timing
They're doing it as a spinoff that new viewers can come into without the previous show. Of course they're starting it the day Aaravos returns. Did we all just forget that season 1 started with Harrow's assassination? It took three seasons to build out all the backstory leading up to that night. They did a similar thing in Arc 1, picking back up where the action did and just flashing back to whatever we needed to know from the lapse. That's likely what they're going to do again. No need to get anyone's knickers in a twist about Missing Everything - they'll tell us what we need to know like they have the other times they did this. Why are people even surprised.
Zym's Rival
This is an interesting story point for them to bring out, and I do think it's possible that Luna Tenebris' heir is the dragon in question. That said, I also got the vibe that this may hearken back to something Much Older, like pre- Sol Regem old potentially. How did the Sun King take the throne? We don't know that either. I think it's too soon to be sure. I will say also though, we're making a big fuss about an "unsuitable heir" when the full sentence we have about it is "Luna Tenebris lacked a suitable heir", which. Could also realistically just mean she lacked any children. "Suitable heir" could be being used as a singular term here, as it often can be in royal contexts. And lacking children would just mean she never actually chose a suitable heir before her disappearance. The one that crossed my mind that I haven't seen mentioned yet is the possibility that it's literally just her, returning, to reclaim a throne that she was taken from.
The Baby
They very well might have been trying to set up Rayllum children in Season 7, and I wouldn't be mad about that. I doubt they would plan on having a child that close to Aaravos' return, but accidents happen, especially in the pseudo-medieval world they're living in. I think that's the theory that has the most active substance from the show so far. However, TDP has been known to throw twists in, so here's the other options I think are most likely:
Janai and Amaya refused to let Aaravos dictate their life choices, and realistically Janai needs an heir now that her siblings are both dead. This is their child.
It's Claudia's child, probably with Terry (whether with his participation or not, given dark magic). This narratively places her flipping the tables so instead of being the abandoned child she is now the parent, and especially if contrasted with any other characters (coughRayllumcough) having their own kids, could produce a strong narrative mirror with her parenting through her madness. I don't think it will be enough to redeem her, and I don't think it will be "Aaravos reborn". Startouch elves aren't mortal beings like humans and other elves, they're constellations taking vaguely mortal form, and we already know what form Aaravos chooses.
Edit to add: It also occurred to me a little while later that it could also be Soren's child, producing another narrative flip from being the forgotten/abused son to being a father. It would be especially powerful seeing him take on that role after so much of his previous arcs have been about his tumultuous relationship with his family.
As a side note, it kind of baffles me that I'm seeing people being upset about the inclusion of a baby in The Dragon King. The Dragon Prince has been about family, the complicated relationships between parents and children, and overcoming generational hatred and conflict since Day One. Our original main characters are the adults of the tipping point - it feels actually fitting to me that the final arc will touch on the children who will actually grow up after it, who won't remember the conflict directly.
