I'm thinking campfire themed?
Like imagine an animal that appears to be made of sticks - but has a burning tail or similar. I want to say rabbit, if only because the word in the rabbit world for a rabbit giving birth is "kindle" or "kindling" - just like the wood you use to start a fire. The plant matter doesn't necessarily have to be alive to be plant matter, after all.
There's also potential for a charcoal pokemon? Charcoal for the large part is just crispy burned wood.
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As for other unused types...
I have a potential idea for a normal/steel type -
An alpaca made of steel wool. Something like a
secretary bird would make for an interesting flying/fighting. Bug/Psychic would definitely be an interesting combo, would love to see that. Similarly bug/dark (Roach?). And anything that hasn't been done with dragon would be good (Poison, Rock, Grass, Bug...) More ghosty combinations, too, please.
I would also prefer more non-dragon pseudo-legendaries in general. Gimmie something more like Tyranitar. I think we went a little overboard with dragons last generation, anyways. Three dragon legendaries, two full chains of dragons, plus druddigon was a Lot Of Dragons. Over a third of all the dragons we have. I'd expect to see a lot fewer dragons in VI.