Perhaps a new character created specifically for the fanfiction. A time - traveling warlord from a far - off future who sees the present timeline as easy prey. He has advanced technology and powers that the Z - fighters have never seen before, and he's set on conquering the world that Future Trunks has time - traveled to.
Cell could still be a great antagonist. Imagine a scenario where in the fanfiction, Cell follows Future Trunks through time. He then starts to absorb different beings in the new timeline in a more devious way than in the original story.
He becomes more resourceful. In his own time, he's used to a certain set of circumstances. But during time travel, he has to adapt quickly to new situations and find ways to use the resources available in the past. For example, he might have to figure out how to use different senzu beans or power - up techniques that are new to him.
It could be a rogue Saiyan from an alternate timeline who has come to their universe to prove his supremacy. He is jealous of the power that Goten and Future Trunks possess and wants to take it for himself.
It could be a future version of Orochimaru who has found a way to come back even more powerful. Maybe he has developed some new form of immortality and is causing chaos in the future world that Naruto travels to.
One possible plot could be Future Trunks traveling back in time to prevent a new threat that he discovers in his own time. For example, a powerful and unknown enemy is about to emerge in his future, and he goes back to enlist the help of Goku and the others before it's too late. Another plot could involve Trunks accidentally changing something in the past that has unforeseen consequences in his future, like messing with the timeline of a certain character's power - up, which then leads to a whole new set of challenges when he returns to his own time.
One idea could be Future Trunks bringing back a technology from the future that changes the course of the present timeline's battles. For example, a more advanced scouter that can detect hidden power levels more accurately.
There's also the possibility that the main antagonist is an outside force from another dimension or a powerful entity that was awakened in the future. This entity might see the ninja world as a threat or simply want to conquer it for its own gain. Naruto would need to figure out how to defeat this unknown and powerful foe during his time in the future.
A lot of Trunks' time travel fanfiction focuses on fixing the past. He has the power to change things that went wrong in the original timeline. It's really cool to see how writers use this concept to create alternate versions of the Dragon Ball story.
Trunks' time travel in fanfiction is a great way to expand on the original story. For example, some fanfictions might explore what would happen if Trunks made different choices during his time travels. Maybe he could prevent certain tragedies or cause new ones. It gives a lot of creative freedom to the writers to come up with unique and exciting plot twists.
It could be a time - traveling villain from the future who wants to change the past to gain more power in their own time. They might try to stop Naruino from achieving their time - travel goals.