I think Tooth Fairy would be the first. She is very kind - hearted and is always interested in new things. Elsa's unique ice powers and her beautiful ice creations would draw Tooth Fairy's attention, and she would be eager to make friends with Elsa.
In a fanfiction where they meet, Elsa could learn a lot from the Guardians. For example, Bunny might teach her about how to better control her powers in a more natural way, as he has a connection with nature. Elsa, in turn, could show them how to build an ice castle in a more efficient and beautiful way. Also, Sandy could communicate with Elsa through dreams and help her overcome some of her fears related to her powers, which could lead to a great friendship between them.
Some Guardians might be initially shocked if they witness Jack getting spanked. For example, Tooth might be very taken aback as she is more of a gentle and nurturing character. She might try to comfort Jack afterwards. Bunny, on the other hand, might think that Jack deserved it if he had done something really wrong, like endangering their plans against Pitch. North might be in a more difficult position. He is the leader, and while he might not fully approve of the spanking method, he might understand the need for discipline.
Well, sort of. The movie doesn't really have a traditional 'pitch black love story' but there are some dark and intense emotional connections between the characters. They are all so different and they have to learn to work together. This process involves a lot of friction and then gradually some form of deep bond, which can be seen as a deviation from the normal love story concept and has some 'pitch black' aspects to it.
The other characters would be worried. North might immediately start making plans to take care of Jack. He could offer him a warm place to stay in his workshop.
In this fanfiction, Elsa might use her ice powers to create amazing ice sculptures for the Guardians. They could be in awe of her abilities and she could learn some new things from them too, like how to better protect the ones she loves.
The idea of good vs. evil is present in both. In Rise of the Guardians, it's the Guardians against Pitch Black. In Frozen, it's Elsa against her own fears which can be seen as an internal evil. In fanfiction, the Guardians could help Elsa face her fears. Jack Frost could use his carefree nature to show Elsa that she doesn't need to be so afraid. And the Frozen characters could help the Guardians in their fight against Pitch by bringing their own unique skills and determination.
Some fanfictions might take a different turn and introduce new threats to the guardians. Picture a story where a powerful dark force from another dimension comes to their world. The guardians then have to find new powers within themselves and work together in ways they haven't before to defeat this new enemy.
You can start by looking on popular fanfiction websites like Archive of Our Own or FanFiction.net. Use the search function and type in 'Guardians Watches Rise of the Guardians'. Then, you can sort the results by popularity or rating to find the ones that are highly regarded by other readers.
The most interesting pair could be Elsa and Bunny. Imagine Elsa's elegance and Bunny's gruffness. Bunny could teach Elsa about the magic of the Easter eggs and how they represent new life, while Elsa could show Bunny how she can create beautiful ice art. Their different personalities and powers would make for an exciting dynamic.