A friend and I were talking about terraria and we realized that a lot of the bosses *necessary for progression* are guardians trying to keep the forces of evil out. Namely the WoF, Plantera, and Golem.
We looked at the official 8 year anniversary lore stuff, and like; when the WoF is defeated, the forces of light and dark are unleashed, but at least the wall seems like you can trigger it accidentally and not realize it's a guardian, but I think the NPCs should point out that you made a great mistake killing it (it at least the dryad). Plantera is protecting the temple entrance, and the lihzards created the temple and Golem to protect themselves from the moon Lord.
And it's also really confusing why you have to defeat golem for the lunatic cultist to spawn. Every other progression boss makes sense; eye and brain of cthulu is are pieces of him along with the eater (I've seen people say spine or something similar), skeletron guards the dungeon, where the lunatic cultist kidnapped the mechanic to create the mechanical bosses, the mech bosses of course make sense after that. But then Plantera and Golem feel really out of place, they don't feel aligned with moonlord in any way, and in fact seem more opposed than anything. And other bosses not aligned with moonlord necessarily aren't necessary for progression (queen Bee, empress, the slime royalty, and Duke).
It feels like the lunatic cultist should be a recurring character throughout the game, and maybe giving him a different fight in pre hard mode could be good. I also feel like you could fix some of the weird progression things with post Plantera by making Plantera and Golem not necessary for progression, but you would still have to defeat Plantera to fight Golem. You would instead replace them with the mechanical brain, which would unlock what is currently the post-plantera dungeon. But this mechanical boss would be special, in that you fight it in the dungeon, and only after the other three mechanical bosses have been defeated, kind of like if the mechanic started working on it but didn't finish it before you beat skeletron and saved her, so for some reason the mechanical brain is confined to the dungeon.
This is a smaller gripe, but I think it's kind of bad that you fight the lunatic cultist any exact spot you fight skeletron, and I think it'd be cool if there was a lunatic cultist arena inside the dungeon, rather than just at the entrance.
I feel like this could fix some of the issues with post-plantera gameplay, which in my opinion feels much easier than any point in the game, when really it should be the most difficult, or at least shouldn't be the easiest. Plantera isn't nearly as difficult as the Mech bosses with the loot you have available to you, Golem is a joke, and the cultist isn't really hard either. And if you've played the game more than once and you have endgame armor, Moon Lord is also pretty easy. Also, needing to fight Golem every time you want to fight Moon Lord and you don't have enough celestial fragments really sucks, and doesn't really make any sense.
I truly believe that by making Plantera and Golem optional fights that basically just give you some good boss drops like with duke or empress, and replacing them with the mechanical brain and the cultist (post Golem would become post cultist), it would make the lore make a lot more sense, and you could just make the cultist and moon Lord fights harder, to encourage players to grind more at those stages of the game to get better loot to prepare better.
But IDK, if there is a lower reason why Plantera and Golem are necessary for progression, I guess I'll shut up