I've talked a little about this before, and I still feel mostly the same way.
https://worstgen.alwaysdata.net/forum/threads/the-4-deities-of-ancient-kingdom.82516/#post-6715791
The real x-factor here is that most of the God Fruits that we've seen haven't really had much ties to their actual element. Luffy's powers aren't heat or light based like you'd expect for a Sun God, he's stretchy and uses his "imagination" to create things.
So...I'll just bring this up...do you notice that Zaza has 4 arms?
Kind of reminds me of...Robin.
Could ROBIN'S Fruit actually secretly be the Rain God Fruit?
I mean, we did see Robin go into a Demonic form with Demonio back in Wano. And, as we've just learned, there isn't that big of a difference between a God and a Devil, in this world. The Hana-Hana Fruit SEEMS like it'd tie more into plants, and be a candidate for a secret Forest God Fruit, but maybe Oda's giving us a twist here. It'd be weird, but not impossible.
Zaza's also wearing a Japanese-style kasa hat. Which...is technically a STRAW hat. And the style makes it seem like the original Zaza likely had some connection to Wano back in the day. Also, Zaza's limbs are made of water. So, this is the first "Devil Fruit-like" power that we've seen that's able to directly control water. So...just more things to keep in mind I guess. Although this is a "Nightmare" version of Zaza, so it likely doesn't totally reflect reality, like we saw with Nightmare-Nika.
So, yeah, personally, I'm still kind of eyeing Robin to be our "Zaza". It'd kind of make sense for the "Demon" that the Celestial Dragons fear to be tied to one of the most wanted criminals in the world.
Dragon is admittedly a lot more obvious of a possibility, but it seems like the God Fruits just aren't as directly tied to their elements as you would actually expect. So, I'm anticipating a bit of a twist, here.
We're probably looking for someone who's more "metaphorically" tied to rain in some way, rather than literally.
Which is why Perona actually isn't a impossible choice. Rain is associated with "gloomy" days, at least here in the West. Perona's Horo-Horo Fruit makes you depressed, like a overcast rainy day. There's a metaphorical connection here, even it's a VERY loose one. And her "ghosts" could actually be "teru teru bozu" charms, giving her a direct connection to weather. And if we want to say that the Horo-Horo Fruit can actually give you 4 arms, like we saw sometimes with Gil Bastar back during the God Valley Flashback, that gives the Horo-Horo Fruit even more of a connection to the 4 armed Zaza. Although, Oda may have retconned that in the tankobon collections. So, maybe or maybe not on that.
And as for Vivi...I don't think she already has a Devil Fruit, yet, personally. But, if she gets one later, maybe? It works thematically, but I don't think she's already got it. We HAVE seen Vivi swim in the ocean, before, back during the Laboon Arc (Chapter 104), so if she's already got it, she got it between then and the present day.
But, we'll see. Oda could be setting up Zaza because The Man Marked By Flames has the Rain God Fruit, or something. Oda does love introducing brand new characters into the story, after all. So, it could just be someone we haven't met yet. Which is admittedly less fun to speculate about.
...I mean, for all we know, it's actually Luffy's MOM who has the Rain God Fruit, and she's just been fighting beside her husband Dragon this entire time in the Revolution. Just SLIGHTLY offscreen, so that we've never met or seen her, yet. It's not out of the question for Oda to do something like that. Okay, baseless speculation about new potential characters can ALSO be a little bit of fun.