It could be another powerful villain from the Dragon Ball universe like Cell. Since Gohan and Frieza are starting to form an unlikely bond, Cell might see this as a threat to his own plans of domination. He could try to break them apart and defeat them both.
Obviously, Frieza would be a main villain. He is always power - hungry and self - centered. If Gohan has a pet with some special powers, Frieza would want to take it for himself or destroy it out of jealousy.
It could be a new villain created specifically for the fanfiction. Maybe a powerful sorcerer who wants to steal the Saiyan's power.
It could be Beerus. Since he is a powerful God of Destruction, he would see the union of Frieza and Cell as a threat to the balance of the universe and try to stop them.
A powerful dark wizard or sorcerer could be the antagonist. He might be after the Saiyan's life - force or some special energy that Gohan and Bardock possess. This wizard could use dark magic to create all sorts of obstacles for them, like illusions, cursed landscapes, and powerful minions.
One idea could be that Frieza, after being resurrected yet again, finds himself in a situation where he has to team up with Gohan. During their time together, they start to understand each other's motives and past traumas. Gohan's kindness and sense of justice gradually influence Frieza in unexpected ways.
It could be someone who doesn't want Gohan and Zeno to form an alliance. Maybe a rogue god from another universe who fears that if Gohan influences Zeno, his own plans for universal domination will be thwarted.
Cell could be a great antagonist. Even if Gohan goes back in time, Cell might still find a way to exist. Maybe he adapts his plan to absorb the Androids earlier in time. Gohan would then have to face a more powerful and unpredictable Cell, who could potentially change the timeline to his own advantage.
It could be a new villain who is specifically targeting Gohan because of his new form. This villain might see Gohan's transformation as a sign of weakness or an opportunity to mess with the Z - fighters. Maybe they want to use Gohan's confusion and the chaos it causes among his friends and family to their advantage.
It could be Salem herself if the fanfiction focuses on Naruto trying to stop her evil plans. Since Salem has a long history of causing chaos and destruction, Naruto would see her as a force to be reckoned with and try to oppose her.