Yes, there are likely some interesting fanfictions out there. Fanfiction often explores different aspects of the characters and their stories. It might expand on Anna Sackville - Bagg's adventures as a little vampire, perhaps her relationships with other characters in the original story, or it could create new scenarios for her to face.
Friendship is a very common theme. Since Tony and Rudolph are the main characters in this fanfiction concept, their bond and how they support each other through different situations is often explored.
Well, it could be about Rudolph and Tony going on an adventure together. Maybe they explore a spooky old castle that has some hidden secrets. They might encounter other supernatural beings and have to work together to overcome challenges.
They could be best friends. Tony might be the first human to accept Rudolph for who he is, and Rudolph might see Tony as someone he can trust completely.
Rudolph is often cautious and thoughtful, while Tony is more impulsive and bold. Tony's boldness can encourage Rudolph to take risks he might not otherwise take, and Rudolph's caution can keep Tony from getting into too much trouble.
One place to start looking for such fanfiction could be popular fanfiction websites like Archive of Our Own. Just search for 'the little vampire tony x rudolph' on there, and you're likely to find some interesting reads.
They could go on a journey to find a hidden vampire lair. Maybe they'd encounter other mythical creatures along the way and have to use their wits to outsmart them.
Since I'm not familiar with the 'little anna story', it could be a story full of magic where little Anna discovers a hidden world. Or perhaps it's a story about how she overcomes difficulties at school or at home. There are so many possibilities.
Well, one possibility could be that Anna is turned into a vampire in a Frozen - themed fanfiction. Maybe she's bitten during one of her adventures in the Enchanted Forest. This could lead to a whole new set of challenges for her and Elsa. Anna would have to deal with her new vampire nature while still trying to maintain her relationship with her sister and the people of Arendelle.