My group was also at the point that pure vanilla became too easy. Then we tried lightly modded server (just some better foods and enemies up to 3 stars). It was fun but we had some troubles with tweaking mods to our likes. It was still too easy. Enemies still had the same attack patterns and we still used same tactics in same situations. Nothing really changed except the amount of hits we needed to kill them.
Then we decided to go back to vanilla but with NOMAP. Now, this is a proper challenge. Everything is still the same but we needed to change our approach to everything. And there is a lot extra things we have to think about.

If you want to scale the difficulty without changing anything, try using only light armors. Try playing without upgrades to your weapons. Or maybe a play through without a cauldron, so basically you eat only berries and meat cooked on cooking racks. I even played without a bow on one world. There is so much combinations of rules you can set up for yourself.

Limit the gear you use. Lots of combinations there. hardest one ive done is the meadows challenge, will have to try continue with that one and see if mistlands is possible.

No portal is fun I don't see it as harder.

No map however is bloody hard imo. I have been trying a map and am having a real hard time time finding bonemass even after finding an above ground marker.

Originally posted by Red Falcon:

So I'm trying to find some levers to pull like setting the server to a difficulty of as if 10 people are in it at all times; or trying to otherwise increase difficulty through other means such as increasing the raid frequency and try to impose more difficulty on the vanilla game by editing options in the code. Though I am not finding much luck yet, so I was wondering if someone has been able to find some way to do that without the use of mods; so that way people can just load in and not have any need or issues trying to install mods and deal with that sort of mess.

There is a function in the game code, that when players are nearby together, all enemies scale up in difficulty, by gaining a 4% buff to their damage resistance per person (Ostensibly an HP increase), but I think it caps at 5 people (20%) If you find that line, you could certainly edit it to whatever value you so desire.

It's not practical currently without mods. At most you could set the number of people to scale to, but it's unlikely to be persistent - also any client that is first to load a zone "owns" it and for all I know may not respect the setting anyway.

Difficulty options are supposed to be coming, but as of today we have no idea in what form - we might get genuine granular enforceable server options, or we might get a dumb slider, or anything in between.

Im doing a no protal run right now (with no bow side challenge, since i dont like them anyways) and it has been quite fun, not as bad as it sounds actually.
The fact that farms are so slow make you go for seeds ASAP which means swamp/mountain/plains in troll gear etc.

Another is no metal armor... thats plenty doable especially now, not a big increase in difficulty, but more fun to play.

In general the faster you try to play the more difficult it is, thats one nice thing about Valhaim.

Otherwise, yeah valheim plus probably is the way to go. I dont think you'll find much in console commands, except for like different passage of time (which could include tick rate for regen maybe).

I found the best way to increase my chance of getting killed was switching to lighter armour like the Troll Hide set, then using lower level foodstuffs and minor potions. Now I only wear the Fenris set in all situations which offers slightly more armour than bronze. It's speed bonus really mitigates the lower armour level as it is like having a stamina pool that rarely ever goes below half. I ended up downloading the Valheim Plus mod and dropped the armour rating on my gear because the Fenris set is too much fun to give up.

No boats + ironman. But ironman is probably impossible in multiplayer because of lags.

try playing with no gear :) trust me its hard

and spawn 2 of the boss at same time

Honestly, just installing thunderstore and getting Valheim Plus would be less effort than trying to fiddle around with the console commands. I never used the mod but i'm pretty certain it does allow you to tweak things that you mention with a good amount of control. And i also have never used the console, but i would imagine it being limited compared to your other option, though i wouldn't say mods are messy either, it of course depends on which ones you get.

But if you are exclusively looking for console commands to suit your needs then looking on the wiki should help; https://valheim.fandom.com/wiki/Console_Commands , otherwise that's all i have.