So the Solangelo book needs a villain.
Let's start from the roots.
In The Lightning Thief, we're led to believe in the obvious conclusion of Hades being the villain right from the third or fourth chapter, if I remember right. Then we find out Kronos is the villain, and that Luke is also a villain.
Over the next two books, more than Kronos, Luke earns our hatred so well that we start to wonder if he is the actual villain. And right when we are convinced of it, Kronos and Luke merge.
Then, in the TLO, we begin to see Kronos as a villain far crueler than anything Luke could have become by himself, and though we pity him, we still hate him mindlessly. Until he sacrifices himself and proves himself to be a conflicted antagonist rather than the villain.
So, what do we know concerning antagonists about the Solangelo book?
But we've seen this guy. And he's overpowered: We've already seen what a primordial god can do, anyway.
So we've seen a Titan (Kronos), a primordial deity (Gaea), and gods (the triumvitrate) as our villains. You know what I would've done next, had I been the author?
Take the guy whom the fandom adores the most, whom his experiences have slowly caused to fester him darker and darker: but no one, especially not the fans actually note, who would give the world for the ones he loves; and the ones he loves had been threatened enough.
I think Mark Oshiro and Rick Riordan will give the fandom exactly what we have been asking for, without understanding the consequences.
Yes, I mean exactly what you think I mean.