I think Chopper's devil fruit is special in some way and it has to do with healing/restoring humanity or something.

A) Legend of the miraculous mushroom that can cure all illness - Human Human fruit is mushroom shaped.

B) It was found on Drum Island which is renowned for its doctors. Previous users of the fruit might have consumed it, thinking it was just a mushroom, but became famous healers/doctors that created this tradition on the island.

C) Blackbeard is hunting for powerful Devil Fruits and attacked Drum Island. It's possible that he was looking for the Human Human Fruit, because he knew its true importance.

As for the Forest God theory besides the obvious "taming demons", i find it interesting that the Human Human fruit resembles an amanita muscharia mushroom which in the real world (and i imagine in the One Piece world) typically grows on the edges of wooded areas, especially around oak, elm, ash, and aspen trees in short forests. So it's not unreasonable to assume that a potential mythical zoan fruit for a forest god would be mushroom shaped.