Luigi could also be the one to save her. Although often seen as the more timid of the two brothers, he has his own strengths. His high - jump ability and his ability to sense things that are amiss could help him track down Tatanga and free Rosalina. Plus, he might be more motivated because he wants to prove himself as a hero just as capable as Mario.
Mario would be a likely candidate. He's always been the hero in the Mushroom Kingdom and has a lot of experience dealing with villains like Tatanga. He could use his jumping skills and power - ups to make his way to Tatanga's lair and rescue Rosalina.
Well, in such a fanfiction, Tatanga might be after Rosalina for her unique powers. Maybe he wants to use her connection to the stars to gain control over the cosmos. He could hold her captive in his dark and gloomy lair on his home planet. Rosalina, being a strong character, would try to resist and find a way to escape, perhaps using her Lumas to send out a distress signal to Mario or Luigi who would then come to her rescue.
The Salvatores, Damon and Stefan, are likely candidates. They have always been protective of Elena and would do anything to save her. They could use their vampire skills and knowledge of Kol's weaknesses to stage a rescue.
The other Sailor Scouts would be the most likely ones to save her. They are her friends and comrades in the fight against evil.
It could be a rookie cop who's always looked up to Burgess. This rookie might have noticed something off about Gaines before the kidnapping and starts to dig deeper on their own. They could end up finding a crucial piece of information that leads to Burgess' rescue.
Obviously, Ben could be the one to save Gwen. With his Omnitrix, he has all kinds of powers at his disposal to take on Kevin and rescue Gwen. He knows both of them well, so he would be highly motivated to stop Kevin and bring Gwen back safely.
Sky might also be a savior. He has a strong connection with Bloom and would be enraged to find out she's been kidnapped. He could use his combat skills and swordsmanship to fight Brick and rescue Bloom. Another possibility is that Tecna, with her technological skills, could track down Bloom's location and help in the rescue operation.
Nightwing could also be a likely savior. He was once Robin himself and has a strong bond with the current Robin. He might take it upon himself to rescue Robin, using his own unique fighting style and experience to outwit the Joker. Plus, he has his own network of contacts in Gotham that could help in the search.
Anna could be the one to save Elsa. After all, she has proven herself brave and determined in the original story. She would stop at nothing to rescue her sister from Hans's clutches.
Batman, of course. He's Robin's mentor and protector. He has the skills, the resources, and the determination to track down Slade and rescue Robin.
Pikachu would be a likely candidate. Pikachu is Ash's most loyal Pokémon and has a very strong bond with him. It would sense that Ash is in trouble and would go on a mission to rescue him. Pikachu could use its electric attacks to fight off Gary or any obstacles in the way.