For my own idea, I'd primarily re-use elements from Sleeping Princess in Devil's Castle - with elements from Curse of the Blood Rubies and re-working both a supporting character from Dragon Ball GT and a villain from DBGT: A Hero's Legacy.
(Apologies for the wall of text).
- The backstory would be that Lucifer was a Demon King who was born around 5 Million Before Age in the First Demon World, after Dabura had killed his father Abura and seized the title of Supreme Demon King; Lucifer later finding a wife and having a daughter - Princess Mamba.
At some point, Lucifer attempts to overthrow Dabura when believing that he was at his weakest - since Dabura no longer had the Evil Third Eye and had previously failed to acquire the Demon Realm Dragon Balls as he was defeated in a battle against Tamagami Number Three.
However, even with an army of loyal Demons and other minions backing him up, Lucifer severely underestimated Dabura - who proceed to defeat Lucifer and his troops; killing Lucifer's wife as "punishment" before banishing Lucifer, his daughter and the Ogre Demons from the Demon Realm; using Sir Warp to send them to the "Outside World" (more specifically Universe 7).
- Eventually discovering Earth and deciding to make the planet their new "Kingdom", Lucifer, his daughter and their minions forged a set of mountains into a new home; the mountains becoming known as the "Devil's Hand" and with their "Devil's Castle" been built in the middle of it.
There, Lucifer and Mamba planned to destroy the Sun to plunge Earth into darkness; using Magic to create a gigantic cannon before crafting a larger Diamond to be charged by moonlight over thousands of years. In turn, they also created thousands of cursed red gemstones known as "Blood Rubies", which would deteriorate everything around them and also cause mutations in living beings, such as mutating humans into monsters. The purpose of the large diamond - after been sufficiently charged - would act as both an amplifier and a magnet to "direct" the Blood Rubies' destructive power as a single red energy beam towards a target when all were loaded into the cannon, with this been their way to destroy the Sun.
In 4250 Before Age, they tested the Blood Rubies effects by inflicting them upon the Land of Gurumes, which had previously been a thriving Kingdom. However, the Blood Rubies devastated the land and mutated their ruler into a tyrannical monster driven by a painful and insatiable hunger, with the Land of Gurumes been effectively wiped off the map; Lucifer and Mamba turning the now monstrous King Gurumes into another of their minions.
- However, the destruction of the Land of Gurumes hadn't gone unnoticed, as one of the Kamis of the Past summoned a group of warriors and magic users - led by Kidomo - to stop Lucifer and his plan. In the battle that followed, Kidomo gave his life to kill Mamba, defeat Lucifer and destroy the cannon; Kidomo keeping his body after death and joining the Other World Fighters on Grand Kai's Planet while the Kami of the Past used his power to disperse the Blood Rubies out into space.
As for the magic users (the descendants of some of them later teaching Fortuneteller Baba her magic), they sealed Lucifer and his minions, including Gurumes, in a mural within the castle while Mamba's spirit was magically sealed within the large diamond created for the cannon (earning it the nickname of "The Sleeping Princess") before it was buried inside one of the Asteroids in the Asteroid Belt. With their victory, the castle itself was abandoned.
Over time, the "Sleeping Princess" would be mythologized - leading people to believe that it was actually a woman of royalty trapped in an enchanted sleep.
Now the backstory is out of the way, here's my rough ideas for the main story:
- This 'sequel' to the High School Saga would take place around a year after the events of the Super Hero Saga, with it now been early in Age 784 but before the 28th World Martial Arts Tournament - with Trunks and Goten continuing their heroics as Saiyaman X-1 and X-2 respectively, with them even 'inspiring' the creation of another "superhero"; Captain Chicken.
At school, however, Goten takes notice of a shy girl been bullied (her wearing a hoodie with the hood up and clutching her books tightly against herself) and tries to befriend her, although the girl is reluctant to trust him due to too many negative experiences in the past and thinking that he is planning to humiliate her is she lets her guard down.
- The girl, in this case, would be Valese; Goten's girlfriend from DBGT. However, this version of her would be more of a homage to Carrie White from Stephen King's Carrie. Since Valese in GT already had strict parents and wasn't very knowledgeable about certain things as a result of her neglectful upbringing, I'll tweak her Super incarnation slightly to make it so that she not only has to deal with bullies but also her very strict mother, who rarely spends time with her and even treats her like she's a burden. Additionally, Valese is also dealing with the pain of recently losing her beloved Grandmother, with Valese's Mother and Grandmother having not gotten along while she was alive.
(Other similarities to Carrie include Valese's appearance on a poster showing her as been blue-eyed and blonde - similar to Sissy Spacek and Chloë Grace Moretz's portrayal's of Carrie White - and with her wearing a red dress, as Carrie wore to the Prom in the novel. As for the telekinetic powers, I'll get to that next).
- The plot would get moving thanks to Monaka; him flying towards Earth to deliver some items to Bulma - only to come under attack from a group of alien criminals (I'll just use the Mil Clan in reference to another of Toriyama's works).
As they fly through the Asteroid Belt and the clan attempt to disable his delivery truck with a sonic pulse bomb (since Toriyama was a Star Wars fan), one of the clan notices a large diamond thrown up in the debris. Bringing it onboard and leaving it by the window of their ship, they resume their pursuit of Monaka towards Earth.
However, the diamond begins to glow red as the moonlight hits it before fizzling with lightning that messes with the ship's controls; causing the Mil Clan's ship to crash on Earth in the forests near Satan City at night.
- Nearby, Valese (in the forest to recover her pet dog after he ran off into the forest earlier) witnesses the ship crash to Earth like a meteor and - with some hesitation - heads over to check it out; indeed finding the crashed ship and with the Mil Clan alive but unconscious onboard.
Finding the diamond, however, she picks it up - with it again glowing red in the moonlight before its 'power' seeps into her; the diamond returning to its blue colour before Valese fearfully runs back home. Shortly afterwards, Jaco arrives in response to Monaka's call while he was getting attacked in order to recover and arrest the Mil Clan.
Shortly afterwards, Valese notices that she now has the power of Telekinesis and begins to practice with it; keeping it a secret from her mother as she works out what she can do.
- Goten later learns of Valese's powers when he spots her as Saiyaman X-2 - albeit with her face covered and wearing a different colored hoodie - with her trying to use her Telekinesis to fly, but doing so awkwardly and haphazardly (not Pumaman-level awkward but not very well) while letting out screams and shouts of apology to anyone she nearly flies into or if she hits any of their property.
Goten does try to catch her, but results in both of them landing awkwardly; Valese losing her face covering and Goten losing his helmet, which results in them learning each other's identities. Talking for a while, Goten - remembering when Gohan taught him and Videl to fly - offers the same for Valese; the two training together and forming a genuine bond. This includes Valese not only growing stronger but also more confident - with her sharing more of her past with Goten, who is impressed by her fortitude and begins to cutely crush on her (even awkwardly asking if she'd like to become "Saiyaman X-3" in the future).
This would be a good way to give Goten some much needed character development.
- However, the downside of Valese's new gains becomes apparent as Mamba's spirit begins to properly re-emerge after getting used to been in Valese's body; resulting in a more Jekyll and Hyde situation as Mamba begins to take control (using Valese like a 'time share', similar to this) despite Valese becoming aware of it and trying to take back control from her literal 'inner demon'. (Think a female version of this song as the two of them battling for control of Valese's body).
Ultimately, Mamba is able to gain greater control and even transform her appearance to resemble her original self; working to liberate her father and minions, re-build the cannon and steal the Dragon Balls to use a wish (only one as she's a first-time user) to return the Blood Rubies back to Earth to resume their mission of destroying the sun to make Earth their new "Kingdom". Mamba - using Valese's appearance - would even take Goten by surprise in order to steal some of his blood for her father to drink, thus giving Lucifer his own equivalent of a "Super Saiyan" form to empower him and potentially grant him some immunity to his classic vampiric weaknesses.
- As such, it falls to Goten and Trunks to learn all they can about Mamba, Lucifer and their previous plans - having Spike the Devil Man and Fortuneteller Baba provide the details here - before they and their allies (not decided on who would be helping yet, aside from none of the strongest fighters like Goku, Vegeta, Broly, etc.) launch their attack on Devil's Castle to overcome Lucifer's minions, kill Lucifer, stop the cannon from destroying the sun and save Valese from Mamba's spirit before she attains permanent control over Valese's body.
- Not 100% on everything after this, but we'd see Trunks and Goten finally achieve Super Saiyan 2 (since it's about time they got it) and fuse into a more serious version of Teen/Adult Gotenks (think Gogeta from Fusion Reborn); him using a Stardust Breaker or similar attack to successfully destroy Mamba's evil spirit and free Valese - albeit with Valese still retaining some of her powers.
Aside from his own "Super Saiyan" form, Lucifer would also - out of fury upon seeing his daughter die for good - achieve a "Final Form"; stealing the power of the charged Diamond and Blood Rubies to give himself a more monstrous and powerful transformation, similar to Dracula's "Hellbeast" form in Van Helsing.
- Ultimately the heroes would win, Earth would be saved, Valese would get a happier ending (not only overcoming her bullies and cruel mother but with it been revealed that her Grandmother left her everything in her will, including a large fortune) and she and Goten would resume both training - with Fortuneteller Baba maybe offering to teach her some magic following what happened - and dating.
Valese retaining her powers also reminds me of the three classes for human characters in Dragon Ball Online, with three of the female heroes now each representing one of those classes: Videl representing the "Martial Artist" class, Mai representing the planned "Engineer" class and Valese representing the "Spiritualist" class.
