I don't use content adding mods myself, just a few Quality of life tweaks, but I've seen a few that effectivly completed the Ashlands biom, with flaming skeltons and Fire Drakes. I know there is also one that adds giant Krakken mini-boss to the Ocean. There are probably a lot more available to find once you start looking for them.

Steam and Iron Gate don't officially support modding for Valheim so the game doesn't have a mod workshop at this time. But there is a great mod engine called BepInEx for the Unity game engine that Valheim uses. It is easy to install and manage active mods. Just remember that if you do have a problem with a mod you can't expect help on the official Valheim forums

BepInEX
https://thunderstore.io/package/bbepis/BepInExPack/
there is a install pack that is specific to Valheim but the basic package should work too.

Places to look for mods
https://www.nexusmods.com/valheim
https://thunderstore.io

Good luck and Happy Modding o7

(Oh, and be sure to backup the Valheim game folder befor making any changes. I've never had problems with BepInEX but it's just good practice to be safe.)