The concept of Anna getting fire powers in a fanfiction is really interesting. It could be a result of some ancient magic that was dormant in her all along. Her fire powers would contrast beautifully with Elsa's ice powers. For example, they could team up in a new way, with Elsa creating ice structures and Anna using her fire to add details or melt parts for functionality. Also, Anna's fire powers could symbolize her inner strength and passion that was always there but now has a physical manifestation. This would open up a whole new world of adventures for her and new storylines for the Frozen universe.
Sure. One popular 'anna gets powers fanfiction' might be on platforms like FanFiction.net. You can search for it there using the exact phrase 'anna gets powers'. There are often many different takes on how Anna gets her powers, whether it's from a magical artifact or some sort of genetic mutation in these fanfictions.
A dark entity from another dimension could be the main antagonist. It senses Anna's new powers and wants to use them to open a portal between dimensions so that it can bring its army of monsters into Anna's world. So it constantly hunts Anna down, trying to either force her to use her powers for its own evil plan or simply kill her and take the powers for itself.
There could be a fanfic where Anna gets ice powers and she embarks on a journey to discover the origin of these powers. Along the way, she meets new characters and has to overcome various challenges using her ice powers and her strong will. This would add a whole new adventure aspect to the Frozen universe.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, including Frozen. Just search for 'Anna gets ice powers' in the search bar and you'll likely find some great reads.
In fanfiction, these fire powers might be connected to the magic of Arendelle in a deeper way. Perhaps they are a part of the kingdom's forgotten magic that is now resurfacing through Anna. This could lead to her uncovering secrets about Arendelle's past and using that knowledge to better the future of the kingdom.
One way this could be explored in fanfiction is by having Anna struggle to control her new fire powers at first. Maybe she accidentally sets things on fire when she gets emotional. But over time, with the help of Elsa or other friends, she learns to master it and uses it for good, like protecting Arendelle from new threats.
Perhaps an unknown ice sorcerer. Since the Frozen world has magic, a new character who is jealous of Elsa's power could target Anna to get to Elsa. They might use their ice powers to create dangerous situations for Anna, like making the ice floor slippery and causing her to fall and get hurt.