I've been enjoying the game so far and like what I see both in the progress made, and future content coming up. So I thought I'd give my own idea into the mix. Hel would be a boss, a biome, and bring the 'invasions' more flavor.
1 – When Hel begins
Hel notices you when you kill the first mini-boss, the gatekeeper.
2 – world effect
in keeping with valheim's theme of progression a couple of things mark the stage. Hel shows up in a similar fashion to Odin to watch you, but she has a dangerous look more than odin (who looks like a grim reaper haha). Then when you look up you periodically see a dragon or sky serpent circling the branches of the world tree.
3 – Invasion
Gaining hel's attention allows for special invasions where an old boss you have defeated already shows up in spirit/skeletal form to attack your base. Higher health and damage follows naturally, and they drop 'Spirit Ashes'. Spirit Ashes would be special coal that only burns the materials you gain in Niflheimr and when applied to a fireplace actually decreases illumination rather than increasing it.
4 – Entering Niflheimr
One or two 'well' looking structures first appear on the map in a random meadow or swamp. But they have stairs leading down. Go down and you can enter Nnifheimr. An alternative, and really the primary method if you arent a huge explorer, is that the starting altar with all the forsaken stones turns into a descending well at its center upon killing the fifth boss.
Nfilheimr is a foggy shadow land with spirits wandering about as scenery. The first time you enter spawns a large island. The land has day/night but its reversed. The night offers gentle illumination while the day actually makes the fog so bright you can't see far. You cannot build portals, chests, or work stations, though periodically you can find ruins to take shelter in and utilize what is already present. (never a workstation) There is no resting buff at any point but the fireplaces can repel spirits while lit. A lit fireplace with spirit ashes will actually lessen the light by darkfire to make it easier to see through the fog. Periodically you come across chests with random stuff. super random. can include lesser gear and weapons. trophies. wood. Your map doesn't work. There is a giant castle you cant enter.
periodically the spirits turn hostile. at this point, you can hurt them. Spirit damage has a weak poison bonus that, instead of hurting your health and stamina, decays your gear. They don't do much health damage, but watch out because repeated engages means your armor and weapons wont last. they can drop valuables like rubies, etc.
You can't really gain any kind of resources here, no wood/metal/stone/food, outside of chests you come across, but eventually, you come across a miniboss. Killing the miniboss brings Niflheimr to its primary phase as Hel notices you. the miniboss drops a broken key.
When you gain Hel's attention the first time you are sent back to the primary world and the gates are closed. the only way to unlock them is to craft a key. Every invasion with a spirit boss of someone you already killed drops enough ashes to craft one key. You can also keep an eye out and shoot Hel when she appears to watch you like odin and she will disappear while dropping a little bit (not enough, but a tiny bit incase you don't have quite enough.) Combine the spirit ashes with the broken key to heat it enough to reforge it into a repaired key.
You can use the key to open the gate, but it breaks on each use so you gotta keep repairing it and know for certain you want to take the journey.
Every journey unleashes a roaming mini-boss. Kill it and you gain enough spirit ashes to repair your key again or a special material you can use to craft gear and weapons on this new tier. You also get thrown home and the gate is closed… again. So either use the spirit ashes with the boss-material (which will still need different ones or several of the same) to make equipment or use it as a free key repair.
In Niflheimr is altars or dungeons to open the major bosses. they also drop mats, but unique ones you only need one of to make the special armor alongside the slightly more plentiful mats of the mini-bosses. Bosses such as Ganglati, Ganglot, Niddhog, Jotunn, and whatever else they can come up with. Then when you have killed each boss and placed their trophies on the front gates of Hel's castle, you can enter, confront her as a giant dungeon instance, and boom. she either dies or submits.