Star would be an obvious choice. Their different approaches to magic and their positions in Mewni society could create a lot of interesting interactions. Marco could also be a good character to pair with Eclipsa. His down - to - earth nature could contrast with her more otherworldly and mysterious self. And Moon could be another. There's a lot of history between them, and exploring their relationship further, whether it's a begrudging alliance or a slow - burning friendship, would be great.
Another interesting plot could be Meteora forming an unlikely alliance. For example, she teams up with a group of misfits who are also on a quest for something. Together, they face various challenges and in the process, Meteora learns about friendship and trust.
A powerful dark wizard from another dimension could serve as the main antagonist. He might be interested in Meteora's unique powers and wants to use them for his own evil purposes. So he constantly tries to capture or manipulate her throughout the fanfiction.
I'm not sure specifically as I haven't read that particular fanfic. But generally, in 'Star vs the Forces of Evil' fanfics about Eclipsa, they might explore her backstory, her relationships within the show's universe in new ways, or create new adventures for her character.
It could be an ancient, powerful demon that has been awakened by the chaos in the magical realms. This demon might have a vendetta against Eclipsa's family specifically and wants to destroy her line.
One idea could be that Eclipsa discovers a new, hidden power within herself that gives her an edge against the forces of evil. Maybe it's a long - lost spell from her ancient lineage. Another could be a team - up between Eclipsa and some unexpected allies from different realms to fight the forces of evil.
A great plot idea could be that Eclipsa discovers a hidden power within herself that is both a blessing and a curse. She has to figure out how to control it while also dealing with the politics of Mewni. Another could be a time - travel fanfiction where Eclipsa goes back to key moments in Mewni's history to change things for the better. And one more idea is that Eclipsa starts a school for different kinds of magic, which causes a stir among the traditional magic users in Mewni.
You can try searching on popular fanfic websites such as Archive of Our Own (AO3). They have a large collection of fanfics for various shows, including 'Star vs the Forces of Evil'.
One interesting fanfiction could be about Meteora and Star Butterfly teaming up to fight a new, powerful villain that neither could defeat alone. They might start off as reluctant allies but grow to respect each other's powers and abilities as they face challenges together.
There could be a fanfiction where Meteora has to face a new, more powerful threat in the 'Star and Forces of Evil' universe. This would put her in a position where she has to use her skills and intelligence to save the day. For example, a mysterious dark force from another dimension invades and she's the only one who can stop it. It would be full of action and adventure.