All of the characters stay the same in terms of personality and background except for Stella and Octavia, Striker and Stolas change too but to a lesser degree.
Instead of making Stella a torture loving psycho from the beginning, make her start as a bratty and spoiled but overall naive and clueless to the real world child, with a fairy tale view of how her life is supposed to go. She's still arranged to Stolas as a kid, but in the opposite of Stolas' reaction, she's excited to be a princess because her parents have been hyping up that idea to her since she was a newborn, filling her head with promises that she'll be the most powerful Goetia princess some day.
A time skip later, she meets Stolas and realizes things are not going to be quite as she thought, she's been married off to a homo who doesn't love her as much as she tries to love him, she tried her best to get along with Stolas in an attempt to cling on to her fantasy but it just never worked. She still has to produce an heir so now she's stuck in loveless marriage with a child she didn't want while still trying to hide the situation from her wealthy friends and other demon royalty. They argue sometimes here and there, but things are not violent yet. Yes, she's a princess now, but she's still a brat who wants things to happen exactly as she wants them to.
Then you add the rest of the plot with Stolas cheating on her with a lower class creature. This makes her snap out of her fantasy for good. It's not only the cheating that gets to her, because she attempted to make the relationship work as much as she could, but also that he would rather fuck a peasant over her
(Only Stella knows that Stolas is cheating on her with an imp at this point in the story, I'll explain why later). Putting the final nail in the coffin and making her grow into a spiteful, horrid woman who with the years, would start to research ways to vanish and then kill Stolas, not only to claim his riches, legions and powers for herself, but also as payback for all the years she wasted on him and their failed family. She still keeps up the functional couple act and is not as aggressive towards Stolas as she is in the show, but only so no one can find her a culprit later on. If Stolas discovers her true intentions, he'll divorce her and she'll be powerless.
To keep things balanced, Stella has no OP Goetia magic at first because her strength comes from her acting skills, her connections and her manipulative words. Make her use Octavia as a mean to her ends in a proper way, give us scenes of Stella and Octavia doing mother-and-daughter activities and what not, Stella acting like the poor victim around Octavia and telling her she's the best thing that ever happened to her, all lies, she feels nothing for her, she never wanted to have a child with Stolas after knowing how he really is. All of this while Stella plants seeds of doubt and distrust for Stolas in young Octavia's mind. Make Octavia believe that she loves her mother more than Stolas and have her slowly discover that maybe not everything her mother said about her father is true as the series goes on.
Stolas' assassination attempt by Striker happens and it's stopped by Blitz like in the series, but it wasn't a hit ordered by Stella, instead, he's a rogue, stealthy and vengeful assassin who managed to steal an angelic weapon off of a weapons delivery in the pride ring
(Making a direct connection to HH, and reminding the viewer that both of these series do in fact happen in the same universe), As for why Striker is targeting Stolas in specific, have him seek revenge for his family's death at the hands of one of Stolas legions, Stella, who was in the festival's audience happens to see Striker and his angelic weapon in the distance, she then contacts Striker and lures him into her plan with promises of extreme wealth after Stolas dies.
Have Stella find out through Octavia directly that Stolas lends his book to Blitz, Stella fakes a failed assassination attempt on herself in a royal party with the help of Striker, It all goes according to plan. The party could be Ozzie's party that he organized not only to celebrate that Fizz is free from Mammon, but also to give Stolas the asmodean crystal in secret, making Stolas and I.M.P being present at the party as "special guests". Our gang flee the scene and Blitz and Stolas can have their gay break up scene and separate like they do in The Full Moon after they escaped the party.
Stella was invited to the party because in the public face and for all that Ozzie knows, Stella and Stolas are still a happy, normal couple. This means making Stolas cheating on Stella with Blitz not public knowledge until the trial. A way to pull this off i thought about that in the OZZIE'S episode, Blitz would ask Stolas to magically disguise himself as an imp under the excuse that it turns him on or something, when in reality it's to go around without bringing much attention to them, and Stolas gladly agrees because he's too excited that Blitz asked him out in the first place. The bouncer let them skip the file because imp-Stolas targets him with a Goetian demon magic power spell to intimidate him.
Stella calls for a trial, the gang gets arrested and brought upon into the tribunal, the trial begins. Stella blames Stolas and I.M.P for trying to kill her and for illegally using the book in I.M.P's benefit, Stella also uses the opportunity to reveal Stolas was fucking an imp for extra humiliation upon him. If you want to keep that Stolas isn't present at the start of the trial, make her say that he ran off to hide in the human world with the book and thus he's now a fugitive of the law and out of reach when in reality he's off somewhere else still coping with his breakup. Striker appears and gives his false testimony, giving Stella's version more credibility. Remember how Stolas knew that Blitz was in danger in Truth Seekers and it's never really explained how he knew that? That same thing happens here just as Blitz is about to get axed, then you can have the song about how their lives are connected through the power of true love
. Stolas pleads guilty to being the mastermind behind the plan, saves Blitz and gets himself vanished, his Goetia magic powers are given to Stella and he gets kicked out, Octavia wasn't at the trial, but saw it all happen on TV at the mansion.
In Sinsmas, the first half of the episode happens like it does in the real show, Octavia is still with her mother and Stella has taken away her phone, saying that she wants to spare her the pain of his father's betrayal, but Octavia at this point has already seen through the cracks. For the final fight of the season, instead of Andrealphus being humiliated, it's Stella not knowing how to control her new powers yet but still dominating the battle. Octavia shows up and breaks up the fight, but instead of going back with Stella, she finally understands what really happened, fucks off and runs away from home with the book. I.M.P together with Stolas find themselves surrounded by Stella's newly adquired legions of demons who she called so they could finish the job for her, our gang now actually escape to the human world to save their lives using Blitz's crystal.
Stella doesn't care that Octavia ran away, as she already got what she wanted and has no more use for her, With Octavia gone and Stolas vanished, the next step of her plan is to eliminate Stolas once and for all, She puts out a public bounty on I.M.P and personally tasks Striker to take out Stolas. Striker kills some poor Succubus and steals their crystal to gain access to the human world so he can begin his work. Only after Stolas dies, she can keep everything to herself and finally become the version of the princess she was always meant to be.
Now that Stella is a real threat to the gang, Stolas and I.M.P are on the run, jumping through hell and the human world using the crystal to avoid detection and capture, sometimes taking clients in hell and sometimes doing shenanigans in the human world to get by. Maybe now you can shoehorn Vassago wanting to help Stolas here, as he was at Ozzie's party the night of the fake assassination attempt and saw what really happened, but he just couldn't rally support for the gang to defend their innocence.
Away from her mother's influence. Stolas might just have a chance to reconnect with her daughter again.
