It could be rogue ninja from a hidden village. They might be after Tenten because she has some sort of valuable information or a powerful weapon in her possession. Naruto gets in their way to protect her, and these rogue ninja use their forbidden jutsus to overpower Naruto.
One possible plotline could be that Naruto and Tenten are on a mission together. Enemies attack and Naruto, being the selfless hero, puts himself in harm's way to protect Tenten. Maybe they are in a village that's being invaded, and Naruto uses his Rasengan to fend off multiple attackers while Tenten is injured or pinned down. In the end, Naruto sacrifices himself, leaving Tenten with a deep sense of loss and a newfound determination to carry on Naruto's will of protecting the village and its people.
One possible antagonist could be a rogue ninja from another village. They might target Tenten for her unique weapon - handling skills, thinking they can use her to create powerful weapons for themselves. Naruto would then have to face this ninja to save Tenten.
One possible antagonist could be some of the traditionalist ninja in the village who oppose the marriage. They might think that Naruto, as a jinchuriki, is not a suitable match for Tenten. They could try to sabotage their relationship by spreading false rumors or creating difficult situations for them during their missions.
Tenten would likely be deeply traumatized. Naruto was not only a comrade but also a symbol of hope and strength. His death in protecting her would make her feel guilty. She might start to question her own worthiness of being saved at the cost of Naruto's life.
In the fanfiction, Naruto might be targeted by a powerful enemy who wants to steal his Nine - Tails chakra. Tenten, being a skilled ninja, steps in to protect him. She could set up all kinds of traps around their hiding place using her weaponry skills, and engage in combat with the enemy when they get too close, buying time for Naruto to regain his strength or for backup to arrive.
Maybe some of the historical villains who were thought to be dead. If Naruto and Tenten's time travel disrupts the natural flow of time, it could bring back villains like Orochimaru in his prime or some ancient evil ninja clans that were long forgotten. They would see Naruto and Tenten as threats to their re - emerging power.
One way this could unfold is that Naruto and Tenten are on a mission together. They are suddenly attacked by a powerful and unexpected enemy. Naruto, being the selfless hero he is, sees the enemy about to strike a fatal blow on Tenten. He jumps in front of her without hesitation, taking the full brunt of the attack. His last moments could be him telling Tenten to keep fighting and protect the village.
Some fanfictions might introduce new, original villains. For example, a dark sorcerer - type character who wants to use Naruto's unique chakra for his own evil purposes. Kakashi has to face this sorcerer and his minions to keep Naruto out of harm's way.
It could be a group of outcast ninjas who have a grudge against Konoha. They see Tenten as an easy target since she is often on solo missions or training alone. These ninjas might be using cruel and unethical methods to try and break her spirit or get some sort of advantage over Konoha.
It could be a group of outcast ninjas who have a grudge against the Hidden Leaf Village. They target Tenten as she is an easy target for them to vent their anger on. They might be jealous of the Hidden Leaf's prosperity and think by abusing Tenten, they can send a message to the village.