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I dig Meganium altogether, but Typhlosion and Feraligatr might need some work anon.
For Typhlosion, I think the dark fire-type Pokémon has kind of played itself out—why not a rock-fire type, with a lava/volcano/obsidian theme to keep consistent with Hawaii? The blue fire thing may or may not work, as well, because I feel like that's been explored in previous gens with Pokémon like Charizard-X and Alola Marowak. I think red fire would work best with this idea because that's kind of the trademark of the Cyndaquil line imo.
And I don't know how I feel about the water/ice thing because I don't know if works for the original character design. In general, I don't know that I'd buy a reptile like Feraligatr as an ice type given reptiles are cold-blooded, so it wouldn't logically work. As far as design, what about something like Pic Related, a needle nose crocodile native to Hawaii? If you really wanted to add another type to something like this you could make it, say, ground-type, to play with the meta, or fighting-type to make it more of a predator underwater, or even dragon-type, potentially.
Meganium I like a lot, because I think the design is noninvasive and very minor of a change, while still interesting—that said, I'd consider making it Dragon-type, as some other anons suggested, because I think that is more interesting and weirdly more fitting that Fairy type for this design. My thinking is that dragon-type is closer to the tropical flavor you're looking for, especially considering Alola Exeggutor, than Fairy-type. YMMV on that though. And if you make Meganium dragontype then either don't make Feraligatr dragontype, or DO make all three dragontype and make that the gimmick.
Altogether I think your ideas are pretty solid, especially on Meganium and Typhlosion, and I think your instincts are pretty good, but if you continue brainstorming I'm sure you'll have even more interesting ideas regarding these guys, man.
