Ben Tennyson and Tommy Oliver would be a great combination. Ben with his Omnitrix and Tommy, the experienced Power Ranger, could share their knowledge and skills. Tommy could teach Ben about leadership and teamwork in the context of the Power Rangers, while Ben could show Tommy the different alien abilities from the Omnitrix. They could also combine their powers in creative ways to defeat enemies.
It could be Ben when he transforms into Alien X. Alien X has near - omnipotent powers, like being able to manipulate reality. In a crossover, he could use these powers to easily defeat most enemies. However, the Power Rangers also have their strong Megazord combinations. If they could find a way to counter Alien X's powers, they might stand a chance.
Another possibility is a combination of all the Power Rangers morphing into their Mega - Zord form while Ben uses the Omnitrix to transform into a powerful alien like Way Big. The combined strength of the Mega - Zord and Way Big could be unrivaled. They would be a huge force, able to take on any villain that dares to challenge them, with the Zord's size and strength complementing Way Big's power and abilities.
Ruby and Dallas would be really interesting. Ruby's energetic and somewhat naive nature could contrast well with Dallas' more grizzled and experienced personality. They could learn from each other during heists, with Ruby teaching Dallas about the power of Semblances and Dallas showing Ruby some of the more underhanded tricks of the trade.
You could write a fanfic about a character from Ben 10 getting lost in the Power Rangers' universe. Maybe Ben himself accidentally activates the Omnitrix and is transported to the world of the Power Rangers. He has to figure out how to get back home while also helping the Power Rangers deal with their own threats. Or, a fanfic where the Power Rangers are shrunk down and end up inside the Omnitrix, and they have to work together with the alien forms to escape.
I think Naruto himself would be very interesting. His energetic and determined personality would blend well with the Power Rangers' spirit. He could bring his unique ninja skills and transform them into something new when combined with Ranger powers.
Sasuke would be really interesting. He's already so powerful in the Naruto universe, and if he got Power Rangers' powers, he'd be unstoppable. His brooding nature combined with ranger powers could create a really complex and cool character.
Another cool combination would be Bulbasaur and Wonder Woman. Bulbasaur's plant - based powers could be used in creative ways in combination with Wonder Woman's lasso and super - strength. Maybe Bulbasaur could create vines to help Wonder Woman in combat or she could protect Bulbasaur while it uses its powers to heal or create barriers.
Naruto and Owen would be really interesting. Naruto's energetic and positive nature combined with Owen's love for food and fun could lead to some hilarious situations. They could be partners in a challenge where they have to cheer up a whole village using their unique skills.
A fanfiction plot could be about a new threat that is specifically targeting the Omnitrix and the Power Rangers' power source. Ben and the Rangers have to figure out how the enemy is doing this and stop them. Along the way, they discover a hidden connection between the two power systems that was unknown before. There could be some character development for both Ben and the Rangers as they struggle to protect what they hold dear.
One possible plot could be that the Omnitrix somehow gets mixed up with the Power Rangers' technology. Ben, as one of the Power Rangers, has to figure out how to use both the alien powers from the Omnitrix and the Ranger powers to fight a new, super - powerful villain. For example, he could use Heatblast's powers in combination with the Red Ranger's sword to defeat an enemy that is immune to normal attacks.