Interesting, I like it. Needs a bit of work, though - I'm concerned about how to summon Duke Fishron, for example. Maybe make it so the Truffle Worm only spawns in mushroom caves in Jungle worlds, while Mushroom worlds will spawn a
Datana Major larva(you can view it
here, just beware of standard larva grossness) in jungle caves. Both of them are functionally identical and will summon Duke Fishron when fished in the ocean. Also, the honey substitute in the BFG caves seem a bit lackluster and outright inferior to beehive honey, since it only inflicts slowness instead of regeneration. I propose having Mushroom "honey" being turned into the fabled 4th liquid labelled "Spore fluid" and behaves like honey, except it boosts your Mana regeneration instead of your Health regeneration. You should probably be able to convert honey into spore fluid and vice-versa without having to create new worlds and hope for a Jungle world, using materials found in pre-existing caves of the other major biome.
Additionally, after defeating Plantera/Fungus Among Us, you should be able to "Pick World Nature" when generating new worlds allowing you to select the Jungle or the Mushrooms as the major biome, much like how you can "Pick World Evil" between Corruption or Crimson after defeating the Wall of Flesh. Oh, speaking about Fungus Among Us, I believe the pink Faerie Rings you have shown seem too close to normal mushrooms. Plantera's Bulbs are generally much bigger and give the idea of something you shouldn't break(as well as something that needs a pickaxe to do it), while the pink mushrooms are small enough that someone could easily mistake for a new crafting ingredient, or something that can be broken with projectiles(or swinging weapons). I believe a larger, almost-swollen pink mushroom or truffle would work better.
Other than all that, though, this is a definitely interesting suggestion and I would love to have mushrooms as a predominant biome due to their bioluminiscence and constant nighttime.
Yes! Another Steampunker! And the Mushroom stuff is cool too I guess. 
Seriously though, this is a great suggestion. I've always wanted the Mushroom biome too get more love from the devs, and I think this could even fit into 1.4.7 or something along those lines very well, with a little polish. Though we mobile players won't get it until 2020... Our lack of 1.3 aside, keep working at it, and I'd say there's a pretty good chance of you joining the ranks of fans that helped with 1.4, along with that one person whose name I can't remember who suggested spider furniture. Good luck! 
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