Well, a cool storyline might be that the villains from the Ben 10 universe somehow find their way to the Rosario Vampire world. And they start causing trouble by trying to take over the school or capture the vampires for their own evil purposes. Ben then has to team up with Tsukune and the other students to stop them. Or perhaps there could be a story where Ben accidentally gets turned into a vampire - like creature because of some magic in the Rosario Vampire world, and he has to figure out how to reverse it while still using his alien powers.
In a Ben 10 and Rosario Vampire crossover fanfiction, there could be a plot where the technology of the Omnitrix gets combined with the magic in the Rosario Vampire world. Maybe Ben discovers a way to use alien powers to enhance the vampires' abilities or vice versa. There could also be a cultural clash aspect where Ben's human - alien experiences are very different from the human - vampire relationships in the Rosario Vampire world. This could lead to some interesting character development for both Ben and the Rosario Vampire characters as they learn from each other.
Chapter 420, "Terrifying Spiritual Energy, Tearing Through the Ship's Shield", is the clearest early answer because it turns Orbital Defense's situation into a concrete plot event. The chapter gives readers a real answer by opening on When Gu Hang proposed to take the initiative to attack the Green Skin... and ending on Even if they're a bit rough, it doesn't matter..
Chapter 471, ", Victory Strategy", is the clearest early answer because it turns Shui Wuheng Star's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Green Skin Orcs, of course, had no need to rule over humans. and follows through with His expedition was indeed an all-out effort by the Iron-Tooth Clan!.
Chapter 342, ", Four Years", is the clearest early answer because it turns Void Shield's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Under this number, only 20% of the troops in the Rage Bear Legion... and follows through with This matter was thus dragged on, right up to now, for a whole year..
Chapter 323, ", Reject Protracted Battle, Quick Attack Strategy", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with Isabel, in her youth, was renowned for her beauty, and now at... and follows through with He needed prompt, decisive battles, and through such victories, to....
Chapter 12, "Pact of Summons", is the clearest early answer because it turns Damien's situation into a concrete plot event. The chapter turns that into something concrete by moving from Insufficient Magic Essence!» The panel was annoying to stare at, even... to Huh?" Damien stared at the thing as it remained immobile, refusing to.... What makes it stick is that the chapter closes with Thank you all!
Chapter 403, "The Iron Teeth War", is the clearest early answer because the buildup finally cashes out in a real clash. The setup is Gu Hang's warship arrived at Korolya., but the pressure keeps building through He inspected the recent restructuring effects of the Alliance Army....
Chapter 23, "A New Threat In The Forest", is the clearest early answer because it turns Damien's situation into a concrete plot event. A useful checkpoint is the way the scene starts with With trees everywhere, there was enough punching to go around. and follows through with You may proceed." He said after a few seconds of silence..
Chapter 372, ", State Religion Also Needs Alliance Control", is the clearest early answer because it pushes Gu Hang into a new phase or setting. The chapter gives readers a real answer by opening on The reforms in Korolya continued to progress. and ending on The most important aspect was their influence over the hearts of the.... In story terms, the chapter closes with Gu Hang believed that this might truly be The Emperor's blessing.
Chapter 476, ", Land Grab", is the clearest early answer because it turns Star Domain's situation into a concrete plot event. The chapter turns that into something concrete by moving from This was certainly a war waged against external forces, and in fact,... to Even if tangible benefits were sought, that was also acceptable..
Chapter 373, ", Alpha-class Psychic", is the clearest early answer because it turns Gu Hang's situation into a concrete plot event. The setup is He decisively spent those graced points., but the pressure keeps building through After levelling up to LV9, even without the additions from subspace,....