Sorry for the late reply, just got back home.
Yes, of course it's just speculation, noone ever claimed to have hard proof that the 3rd sage animals or w/e would be Slugs. It just seems like thats where the pattern is leading. Who do you think is most likely to be in the 3rd Sage village when we know the first 2 are Frogs and Snakes?
You don't always need to be flat out told to be able to know whats going to happen in this Manga. How many of us didn't know Naruto was the 4ths kid years before they flat out told us or showed proof?
Edit: Again, this is not to say that the Slug will be the last sage mode. It's obviously just speculation atm. Hell, it could be nindogs still for all we know. The only summoned animals they have shown more than 1 of are frogs, snakes, and dogs. We've still only seen the 1 slug I think. So you never know. I'm still going to assume that its the 3rd sannin that knows about the 3rd sage village though.
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