With Luffy and Loki being shown to be immune to Imu's Reversi, I feel like that limits it to only a few things.
It could be that the God Fruits specifically are immune to Imu's power. We know Luffy has a God Fruit, and Loki's Nidhogg used to belong to a War God, too. Which would mean Chopper has some sort of God Fruit. I'm kind of hoping not, because like others have said, it'd be a little repetitive. But, it is possible.
Or, any Devil Fruit User is able to "tag" someone out of Reversi. I don't think we've ever seen anyone with a confirmed Devil Fruit get Reversi'd, or even fight someone who was Reversi'd, other than Chopper. Maybe it works that way for the same reason as why most people can't eat more than one Devil Fruit. The Devil Fruit User's own "Inner Devil" interferes with the "Devil" possessing someone who's been Reversi'd. As we saw with the Shadows getting separated from the bodies of those who are Reversi'd. That'd be...a little underwhelming, but there could be lore reasons for why it works that way.
I'm leaning towards any Devil Fruit User being able to do it, myself. It seems like a odd coincidence that most Giants don't seem to have a Devil Fruit. I feel like Oda set it up that way on purpose, in retrospect. Remember, even Hajrudin originally left Elbaph because he was looking for a powerful Devil Fruit of his own. Elbaph was particularly vulnerable to Imu's attacks, just becasue they lacked many Devil Fruits of their own. Despite the random animals that managed to get them, anyway, like Biblo and that Lion-Cat. I guess on Elbaph, Devil Fruits are for animals, not for Warriors or even Scholars.
But, we'll see. There could be some other twist coming that we can't predict because Oda hasn't given us enough clues yet. Oda does enjoy a good twist, from time to time.
