some things to know if your worried about getting new content on current or older maps.

The game only generates the bioms when you create a new world, the actual content of those biomes isn't generated until you get close near them. As you explore the game generates the terrain out to ~500 meters of the player and marks the area "explored" in the world map.

So, if you wish to start a new game but still be able to get the Mislands content later you have some options. First would be to restrict your explorations to one side of the map as much as possible. If a Boss summoner is on the far side of the world go straight to it with as little wandering as possible to keep the surrounding areas "unexplored". Then when the new content gets added start exploring the rest of your world. While this works it does limit your freedom in the game.

Another option would be to use a mod to reset the world. I've used the "UpgradeWorld" mod to great succes since I had already explored most of my rold and didn't want to lose all the effort I had put into it. The mod lets you specify loctions to leave alone, just x, y, and a radius in a config file, so if you have a forward or fort/base in the Plains for growing stuff you won't lose it. Nice part about this option is you can install the mod loarder and just this mod, use it, then disable or remove the mod loader after your done.

For the mod option you'll need
BepInEx moding engine
https://valheim.thunderstore.io/package/denikson/BepInExPack_Valheim/

The UpgradeWorld mod
https://valheim.thunderstore.io/package/JereKuusela/Upgrade_World/
You only need the server commands mod mentioned if you have a server, the mod works fine without it.

You might also find the Map Cordinate mod handy.
https://www.nexusmods.com/valheim/mods/907
The upgrade world mod can add the same feature but I prefer a seperate mod for general use

As I mentioned above I've used the Upgrade mode and it worked perfectly, both my bases and a Drauger Village I turned into a "sausage Factory" were intact after using the mod.

Also remember to make a backup of your world files before using it though, just in case.

Hope this is useful to you.
Good hunting o7