Maybe it's a corrupted Christmas elf. This elf could have been angry about always being overlooked and decides to use his magic in a bad way. He starts causing trouble in Angel Grove, like making people forget what Christmas is about. The Power Rangers would then have to stop him and remind him of the true meaning of Christmas.
A new villain could be an ice - themed monster. Since it's Christmas, an ice - monster could be trying to freeze the city and cancel Christmas. He might be created by an old Power Rangers villain who is still lurking in the shadows but doesn't want to show their face directly.
It could be a time - traveling villain from the future who comes back to the Christmas season in the Power Rangers' time. This villain has some sort of grudge against the Rangers' future selves and decides to start by ruining their past Christmas. He has advanced technology that the Rangers have to figure out how to counter.
It could also center around one of the Rangers having a personal issue during Christmas. Let's say one of them misses their family. Then, the other Rangers and the Christmas spirit help that Ranger come to terms with it. Meanwhile, they also have to deal with a small-scale threat in the city that is related to the Christmas season, like some toys coming to life and causing chaos in a mall.
A shape - shifting villain who can take on the form of anyone, including their friends and family. Adam and Tommy would constantly be on edge, not knowing who to trust. They would need to use their wits and the unique bond between them to figure out who the real villain is and stop it before it causes too much chaos.
Lord Zedd would be a great villain. He's a classic and very powerful. His dark magic and evil schemes could pose a huge threat to both the Jungle Fury Rangers and any other rangers they cross over with. He could be trying to take over both their worlds or steal their ranger powers for his own dark purposes.
A powerful witch - like villain would be great. She could be jealous of the female red ranger's power and beauty. This witch could use dark magic to create illusions and traps for the ranger. For example, she might make the ranger think her teammates are in danger, causing her to make hasty decisions. Another option is an alien warlord who wants to conquer Earth and sees the female red ranger as a major threat to his plans. He could have advanced technology that the ranger has to outsmart.
A likely main villain could be a new character, a powerful alien - ninja hybrid. This villain would have the ability to use both ninjutsu and advanced alien technology. He wants to rule both the Naruto universe and the Power Rangers SPD universe. He would be a formidable foe as he can use Naruto - like stealth and the high - tech weaponry of the SPD villains, forcing Naruto and the Rangers to really combine their skills to stop him.
One great power rangers war fanfiction could be about an alternate universe where the rangers are at war with an ancient evil force that has been awakened. The story could focus on the rangers having to learn new powers and team up with unexpected allies.
It could be an old enemy of the Power Rangers who has a grudge against them. Maybe a villain they thought they had defeated before.
It could be an evil wizard from Harry's world who has found a way to cross over into the Rescue Rangers' realm and wants to take it over. He might see the Rangers as easy targets at first, but then realizes Harry is there to stop him.
The plot could involve the new girl being a long - lost Mystic Ranger. Maybe she has unique powers that no one else has. For example, she could have the power to communicate with magical creatures that are usually hidden from the other Rangers. This would add a new dynamic to the team as they try to figure out how to incorporate her powers into their battles against evil.
A jealous show - dog breeder could be the new villain. He sees the Dalmatians getting all the attention and decides to plot against them. He tries to frame them for causing chaos at dog shows so that his own dogs can win all the prizes.