Really, why don't more things respawning in this game? It just becomes more boring in part of that and people who join later have less to play with or have a tougher time due to no fault of their own. Since Klei have been looking at world generation options, I thought to list some very important things that should and would be preferable to respawn to address these issues.
Following are things that are either required or quite essential and if destroyed, will not be possible to get back in the world. Some things are already respawn, like trees, and you can renew some things like goat herds through hunting suspicious dirt piles. Why this hasn't been done with more resources, I have no idea. Some of the listed can respawn, but in very specific areas whereas a new world would give them to you in multiple places, so for example new players in an old world need to go to caves or find the pig king to get any gold. Rock biome in particular becomes a very empty biome later in the game. For balance, some could be changed to discourage certain interactions with them as well, like pig houses could require only 1 pig skin to build, and you would only get one back when hammered, or have the world-spawned houses be indestructible and mandrakes could respawn once every year.
* Grass tufts
* Saplings and Lunar Saplings
* Pig Houses
* Catcoon dens
* Reeds
* Cactus
* Mandrakes
* Boulders and Gold Boulders
* Bull Kelp
* Beached Bull Kelp (for the weapon)
* Stalagmites (for fossils)
* Beefalo
* Red, Green, Blue mushrooms
* Myserious Plants (might be wrong)
* Lune Tree
* Clockwork Statues and Clockworks (on surface)
Following are not really that needed resource options or mobs for anything in particular, but it removes an option and content from the world permanently when destroyed. With some destroying them permanently being the old way you can interact with them to begin with. To build merm houses you need to be Wurt to make them, and that character isn't even free and you can't pick her on LAN servers, and that's just not fair. Others make areas challenging, but only for so long as they are there and once you kill/destroy them, the game just becomes more boring.
* Tallbird Nests
* Pig Heads and Merm Heads
* Bones (in desert and lunar island)
* Spiky Bushes
* Spiky Trees
* Totally Normal Trees
* Fireflies
* Tentacles
* Undug Graves
* Merm Houses
* Killer Bee Hives
* Hound Mounds
* Anenemy
* Driftwood
* Stone fruit bush
* Moon Glass
* Shattered Spider Holes
* Spilagmites
* Slurtle Mounds
* Rock Lobsters
* Dust Moths
* Mole Burrows (on surface)
* Saladmander
* Carrat (you have an entire event dedicated to this thing that barely even exists!)
Despite berry bushes not being listed in the world regrowth options, I'm sure I've seen them respawn and sometimes in really weird places like savanna. Some other oddities I have found are things that don't appear to affect anything at all yet are just there, like Bunnymen being in the surface World Settings when none ever even spawn on the surface. Does this affect their frequency? If so, that's a weird thing to even have when clearly there are more important things to cover. And does anyone else find it weird that there is no meteor area on the lunar island?