In Tangled and Little Mermaid fanfiction, Pascal, Rapunzel's chameleon, can be a really fun character to include. He could have some really interesting interactions with Flounder or Sebastian from The Little Mermaid. Maximus, the horse from Tangled, could also be a great addition. And of course, Ursula from The Little Mermaid is a popular villain to feature. Writers can create new scenarios where the heroes from Tangled have to face off against her.
I think Mother Gothel from Tangled can be a very interesting character to feature in fanfiction that combines the two stories. She could form an alliance with Ursula to cause trouble. On the other hand, the good characters like Rapunzel's mother and father from Tangled can also be great to include as they can bring a sense of stability and family values to the fanfiction. And Ariel's sisters can also be interesting to write about as they could have their own adventures in a combined Tangled and Little Mermaid world.