Quick answer
I canvassed the internet, Google searched, AI summarized, deep dove into Reddit, and even asked my own community to find the craziest and sometime useful seeds that you need to try in Valheim. And these are just the minimum distances, so you may have to travel way farther than that. You can even add a seed to the world if you have one, or let the game randomize it for you. On every Valheim world seed, there are 10 possible locations each for Haldor, Hilde, and the Bogwitch.
Both onion and turnup seeds are commonly difficult to find. Carrot seeds can be a bit difficult to find since they are not very common, only spawning in this biome. And to start things off, this seed probably has the craziest spawn I've ever seen, shared to me by Barefoot Cookie. The Mistlands, for example, doesn't enter into world generation until about 6,000 m from the center.
Onion seeds, for instance, can only be found in chests across the mountains overworld and come with a tiny loot chance of a mere 12 1/2% chance. the wild carrot seed, distinguished by their white flowery tops, tiny and assuming and about to completely change your diet. Big shout out to her for finding this one. In most seeds in Valheim, you start very close to the world center, but this one flips that on its head.
Seed basics
So, it will be a while before you find its alt biomes. Turnup seeds, on the other hand, are not found in a chest, but rather growing in the swamp's biome. The good news is the Black Forest has everything you need to start producing bronze, so you can get your farm started pretty quickly, and you definitely should once you found carrot seeds. Here, you'll find yourself nearly 2,000 m from it, which has all kinds of complicating implications.
And you can find these as early as 500 m from spawn. So, admittedly, all the attributes of the seed are negative and probably only useful for a challenge style run. With the swamp being covered with so much water, the turnup seeds spawn far less frequently than carrots in the Black Forest for comparison. For one, this is approximately the distance where swamps begin to appear in world generation, and despite it only being a small bit of coastline, you can basically see spiky swamp trees from the sacrificial stones.
The mushroom variant, specifically, can also be found in the Black Forest. Even though it's the only seed I've ever seen remotely like it, so I had to share. And this seed gives you four chances for it in your spawn. To make matters worse, the three Eikthyr locations are nowhere near you, occurring again almost 2,000 m away.
How to use seeds
At the same distance of 500 m, you could also encounter a birch or small tree Meadows. As an alternative, this seed has one instance of each wood house two and six even closer to spawn with backup locations just a little further out than the previous seed. In fact, the Elder, Bonemass, and even Moder are all closer to your spawn than even Eikthyr. The birch, like previously, will consist of a normal Meadows, but with way more birch trees than normal, while small tree will be absolutely covered in saplings.
Another resource whose spawn can be suspect and always needed in huge supply is iron, and sometimes you can get some bad seed luck with swamps being far away or containing limited sunken crypts. What's really interesting about these alt biomes is that many of them can overlap to make really wacky combinations. Further from the world center, the alt biomes get more interesting at 750 m with the menhir alt biome. And speaking of challenges, these next three fall into a very similar category, offering very limited resources, and the third one offering even more.
You can also encounter this alt biome type in the Plains. None have Eikthyr, any wooden structures, almost nothing to speak of. Next, at 1,000 m, you can find peaceful Meadows that will likely make for the optimal base location. Each consists of the tiniest of starting islands, in fact, the smallest I could find in all my research.
Best seed choices
Although, you can't forget the limits of building around the sacrificial stones due to those pesky magical forces, so space will be constrained. What's special about this one is that it blocks 11 other hostile creatures from spawning. If I had to pick my favorite, it would have to be this one for its surrounding larger ring-like landmass and small river outlet to the larger ocean. Beyond that, occurring at 1900 m are the dark meadows.
So few that unless you're lucky to spawn a few boar for the leather scrap, you won't even be able to make a raft, leaving the only viable escape path being a long swim. It has the same small challenging starting island, but beyond that offers all kinds of great base building opportunities throughout. Here you'll find Draugr villages predominantly on lower terrain and have more hostile weather conditions. Despite what might seem impossible at first, with a bit of food and maybe some health ticks, you should be able to escape all three with a simple breaststroke.
These next two are for those looking to make Valheim's newest weapon, the early axes. The black forest, like the meadows, has nine possible variations including many of my personal favorites. All three of these make for excellent starting points for a challenge run, but also function as an easily spawn-proof fortress with limited area to cover. This seed gives you the best chance to find both early, having four of the wood house two structures and two of the six variant extremely close to spawn.
Map tools
Starting at 500 m you can encounter locations of resources like we saw in the meadows, but this case massive blueberry patches and dense pockets of red mushrooms. The one I found happened to also be covered in blueberries. Structure six on the other hand has a chest 100% of the time and has the same 55% chance of spawning the curious axe head. Best of them all at this distance are ruined black forest.
The resource grind in Valheim is serious, but that's where I can help. It's probably worth the iteration too, as this weapon is easily the most powerful metals weapon these days, making the early game a breeze. Any less and the farm won't work, conflicting with the limiting breeding mechanics in the game. Core wood works nicely as an early game material with sufficient strength to support the structure.
Although, finding just those structures doesn't guarantee an axe head. Not the case in this seed. At its peak, place four 2x2 floor pieces and extend the floor one more meter with four 1x1 floor tiles. This one has a swamp on the starting island incredibly close to spawn, and unlike many swamps, it's flush with sunken crypts with a total count of 27.
Tips
Next, build a U-shape out of half wall pieces, leaving a gap at the end. What you could do is use this seed as your iron mine world. This is where the newborn piglets will tumble into the pen we'll build underneath. And here, you'll have two structures to search.
Even make multiple worlds with the same seed to solve all your iron needs. Lastly, place a barrier in between, which marks where we will separate the two breeding animals into their own compartments. Not to mention the second largest possible mountain that can be found in any Valheim seed, commonly known as the hook, is located on the next large landmass over far to the north. To get the animals into position, we'll have to build a ramp with the 26° thatch roofs working best.
Of course, you will still need to sail to both the north and ashlands, but that's the case in all seeds. Again, core wood works well as a temporary supports to extend the ramp. Another seed to consider is this one, which has all three trader locations practically on top of each other. Optional, but not necessary, you can add railings to the side to help keep the boar on the ramp.