Hullo, hollistergurl here.
Some of آپ may know me from the Heroes of Olympus club; some of آپ may not know me at all.
That's fine, I'm just here to share آپ some of my predictions for... a new series from Riordan, perhaps?
For those of آپ who haven't read The Serpent's Shadow yet, GET YOUR EYES ON THAT BOOK AND START READING LIKE CRAZY!
It's epic :D And this مضمون contains some *cough* spoilers *cough*
Just a warning.
Now, I did post this on the HoO Club, but that wasn't in TOO much detail, because it wasn't fully related to the club and I didn't want to go into depth about a series on a different club.
So, here is مزید depth into this article, as آپ will see; آپ can skip over what I already wrote in the HoO Club, یا آپ can read it over again :P

So, there have been some hints to a possible crossover... in all 3 books.
Now, since I don't want to give the story away, I'll only give آپ guys the context.
Amos's brow furrowed as he looked across at the Empire State Building. "Manhattan has other problems. Other gods. It's best we stay separate." pg. 52, The Red Pyramid
Now, that context just mentioned the Greeks of Camp Half-Blood, the gods of Olympus in the Empire State Building... it was only a mention, nothing to worry about, correct? It just sent a flare in your brains, something like,
OMG RICK JUST MENTIONED PERCY :DD
Yes, because we all love our beloved hero from the parallel universe of the Greeks.

Onto the سیکنڈ book, The تخت of Fire.
He کہا Manhattan had other problems—whatever that meant. And sometimes when I looked across the water, I could swear I was seeing things. Sadie laughed about it, but once I thought I saw a flying horse. Probably just the mansion’s magic barriers causing optical illusions, but still, it was weird.
Talking about Blackjack, perhaps?

It also had Romans... and the chapter عنوان for this was "Romans are Evil."

Ah, here we are, the third book, The Serpent's Shadow. This certainly contained some old friends.
Sadie and the other magicians that she and Carter train go to a Brooklyn school-- she calls it BAG.
And guess who's there...
LACY AND DREW! Our fellow Aphrodite campers.
Lacy had warned me about Drew the first دن of school. Apparently the two of them had gone to some summer camp together-- blah, blah, I didn't really listen to the details-- and Drew had been just as much of a tyrant there. pg. 83, The Serpent's Shadow.
Thanks, Sadie, for paying attention :P *sarcasm*

Okay, onto the other piece of info.
There's this magician یا whatever, Setne, and he's an important part of the book [I won't reveal the end to you, how he's important].
Anyways, he says he hates Greeks.
-.-
Setne sneered. "Stupid Greeks. Moving in on our territory. Taking over our gods. I'm telling you, I got no love for those guys." pg. 224, The Serpent's Shadow.

Next, Sadie having a conversation with her mother...
"Probably," She admitted. "But آپ know what to do. Keep teaching the path of the gods. Bring the House of Life back to its former glory. آپ and Carter and Amos will make Egyptian magic stronger than ever. And that's good... because your challenges are not over."
"Setne?" I guessed.
"Yes, him," Mum said. "But there are other challenges as well. I haven't completely lost the gift of prophecy, even in death. I see murky visions of other gods and rival magic."
That REALLY didn't sound good.
"What do آپ mean?" I asked. "What OTHER gods?"
"I don't know, Sadie. But Egypt has always faced challenges from outside-- magicians from elsewhere, even gods from elsewhere. Just be vigilant."
pg. 390, The Serpent's Shadow.

Hold on... it's not over yet.
In the meantime, there are still hotspots of of evil magic and demon activity all over the world that we have to take care of. We've even got reports of unexplainable magic as close as Long Island. Probably have to check that out. pg. 401, The Serpent's Shadow [Last page].

Well... there آپ have it.
Magic from Long Island-- guess what important place lies there?
Yes, Camp Half-Blood.
Foreshadowing... And TONS of it. Riordan would NOT just let that this series go to rest, since one of the main characters' enemies escaped.
Riordan does NOT let something hang like that-- okay, I'm gonna spoil the ending for آپ guys, so I apologize in advance--
Setne escapes... with the Book of Thoth.
Now, as آپ all know, the Book of Thoth has some spells to execrate the most powerful gods, where to find their shadows, etc.
So... this will be a big problem for Carter and Sadie.
Also, I would imagine that the Greeks would be presented as their enemies-- after all, the gods dislike them very much... They, and the Romans, too, were the ones who destroyed the Egyptian age of glory and magic.
So there's gonna be some old grudges, obviously.
So what will happen? Maybe Chaos will rise again... The gods will rise too, since Ma'at rises with Chaos.
Maybe Kronos یا Gaea will rise again in HoO, یا something like that.
Or, my theory is...
Setne will be behind all of this.
He escaped with the Book of Thoth... he hates Greeks... what if the Greek gods could vanish if he execreted their shadows, too?
Unlikely, but آپ know... anything could happen.
Also, he hates the Greeks-- he probably wants revenge, to restore Egyptian glory, and for HIM to be the pharoah.
Which would mean getting the Kanes out of the way-- especially Carter, since he's the current pharoah.
So, IF Riordan does make an epic crossover series [which I think he will], I expect Setne to be behind everything: he was the one that escaped, and he has a grudge against Greeks.
He might try to get the Greeks/Romans to fight against the Egyptians: and such, creating bloodshed... seems likely to me.
Just like old times.
History repeats itself, after all.

Also, the gods of Egypt and Greek are pretty similar...
Like, Anubis = Hades/Thanatos.
Ra = Zeus [both leaders, king of gods]
Horus یا Set = Ares [Horus and Ares are gods of war, Set and Ares are like, the evil ish ones :P]
Thoth = Athena [gods of knowledge]
Etc, etc, etc...

Even the magicians and demigods are pretty similar:
Carter = Percy [both great leaders, main character focus, etc]
Sadie = Piper [both have the power of words: Sadie can speak divine words because of following the path of Isis, Piper charmspeaks. Plus, Sadie already met Lacy and Drew.]
Zia = Annabeth [both girlfriends of main boy characters, both strong]
Walt/Anubis = Nico [well, Walt hosts Anubis, god of death, Nico is son of the god of the Dead... etc]

So, there are the similarities: hopefully these main characters will get along...

On that happy thought, we'll probably have to wait another سال before RR releases some evidence of a possible sequel series... If آپ all remember, HoO was released approximately a سال AFTER PJO was finished. So, maybe Riordan will be onto something in a year.

On this last note, The Serpent's Shadow was EPIC :D