As of ep 105. These are the theories I believe in the strongest, although I am sure I will be wrong about many of them, or at least certain aspects. Theorising is simply just for fun :)

In no particular order!

• Friede is in hiding somewhere, possibly with Diana
• Amethio's family's house is protected with some kind of Pokemon powers that prevent Spinel from spying on what goes on indoors
• Spinel is desperate for someone's approval (his father's?)
• Crave loves Amethio a lot more than he lets on
• Maria did not die in a traffic accident. Something else happened to her that they couldn't tell little Amethio. Maybe it's related to Rakurium research?
• Friede abruptly leaving Exceed and Maria's death are linked
• Friede met Amethio at Exceed once, but neither of them remember it
• As Amethio gathers the Zygarde cells, they'll show him things like they did with Lucius. This way, Amethio will find out where Friede is and/or some intel about Spinel
• Agate is withholding information from Spinel and thinks he's gone off the deep end with wanting to kill Amethio. She wants to let the kids do what they can because she wants them to stop Spinel, although she's not ready to betray him herself just yet
• There's a secret room/item behind Maria's portrait
• Liko will find an Auspicious Armor in Amethio's family's house and use it to evolve her Carbou into Gurenarma (Armarouge)
• Hamber is Lucca's father
• Friede's Lizardon will Mega Evolve at some point (Mega Lizardon X most likely)
• The Black Rayquaza will show up once Zygarde is back in its 10% Forme and they will both help the kids (including Amethio) free the other Six Heroes and defeat Spinel
• Spinel will tell Amethio the truth behind Maria's death during the final battle in an attempt to throw Amethio off and confuse him
• Sango and Onyx will leave the Explorers once they realise the Strong Spheres are dangerous and harming Pokemon
• Indy and Rubela will betray Spinel in some way (by helping Liko & co or not telling him something important related to Amethio)
• Amethio will go with the RVTs to Kumuri Town and talk to that old woman, who will thinks Amethio is Gibeon and they'll be able to get information about Gibeon's past that way
• Amethio will be taken to the Crystal Pool and will get to talk to Maria (and maybe Gibeon perhaps)
• Uruto will somehow be vital to getting Friede back (whether tracking him down or rescuing him) because his lack of connection to Friede will make him realise something the other RVTs won't