One plot idea could be a 'what if' scenario where Arthas didn't fully fall to the Scourge. He and Jaina could work together to heal the wounds of Lordaeron. They might face opposition from both the Alliance and the remaining Scourge forces, but their combined magic and leadership could bring about a new era of peace and cooperation between the living and the undead who can be redeemed.
A plot could also revolve around an alternate universe where Arthas and Jaina are from different factions but fall in love anyway. They have to keep their relationship a secret while trying to find a way to end the war between their peoples. This would involve a lot of sneaking around, secret meetings, and dangerous liaisons as they risk being discovered and facing the wrath of both the Alliance and the Horde.
Another idea is that Jaina discovers a way to travel back in time to before Arthas picked up Frostmourne. She tries to convince him not to go on his fateful journey. This would involve a lot of emotional turmoil as Jaina has to relive her past relationship with Arthas and try to change his destiny while also dealing with the knowledge that any change could have unforeseen consequences for the present.
Their relationship could be developed through shared memories. For example, they could both recall their time as students in Dalaran. Flashbacks of their innocent days together, studying magic and having friendly competitions, could bring them closer. This could lead to a rekindling of their old feelings in the present of the fanfiction.
One plot idea could be that Anduin and Jaina have to team up to face a new, powerful magical threat that is endangering both the Alliance and the Horde. They start off distrusting each other due to past events, but as they work together, they come to understand and respect one another more.
Well, a possible plot could be Arthas, now a girl, embarking on a journey to find a way to reverse the transformation. Along the way, she teams up with other adventurers. She has to constantly battle her own insecurities as well as external threats. Maybe she discovers that this transformation is part of a larger, more mysterious magical plot that she has to unravel.
Arthas and Jaina were once in love. They grew up together in Lordaeron. Jaina was a powerful mage, and Arthas was a promising paladin. However, Arthas's thirst for power and his fall to the Scourge drove them apart. His actions became dark and cruel, and Jaina could not follow him on that path. Their love was a tragic one, as Arthas was consumed by his inner demons and Jaina was left to grieve for the man he once was.
One key event was their growing up together in Lordaeron. It was where their relationship began. Another was Arthas's decision to pursue power more aggressively, which started to create a rift between them. And of course, his encounter with Frostmourne was a major turning point that ended their love as he turned into a servant of the Scourge.
There might be a fanfiction where Arthas uses his Lich King powers in the DxD world in unexpected ways. For example, his control over the undead could be a new and terrifying power in a world full of devils, angels, and fallen angels. He could also have some interesting interactions with the various factions in DxD due to his dark powers.
One interesting Jaina Solo crossover fanfiction could be one where she crosses over into the Star Trek universe. In this story, she might use her Jedi skills to interact with the technology and cultures there. For example, she could have a mind - to - mind encounter with a Vulcan.
I'm not sure of specific 'lemon' fanfiction stories as they often involve inappropriate content. However, there are many great non - 'lemon' Jaina Proudmoore fanfictions that focus on her leadership, her relationships in a more appropriate way, like her connection with the Alliance and her role in the Warcraft lore.
One interesting story could be a crossover where Jaina's magic from Warcraft somehow gets intertwined with the magic in the Harry Potter universe. Maybe she ends up at Hogwarts as an exchange student of sorts. Another could be a story where Harry Potter visits the world of Azeroth and meets Jaina. They could team up to fight a common dark magic threat that is affecting both their worlds.
One interesting plot could be Jaina Solo traveling back in time to prevent a major war. She could use her knowledge of future events and her Jedi skills to change the course of history. For example, she might stop a key battle from happening by convincing the leaders of the opposing sides to negotiate.