I have so few experience of online games. I can't help, sorry.

Make a key control/an option in the menu to become invisible to any other players, while we must leave the game ? This option could make the character 100% invisible, and make impossible any interacting gameplay with him. ( impossible to move or kill, drag, stole items, push, etc...)

Or the same thing wihtout the invisibility but with the same 100% interacting restriction ?

This option in the menu could workd during a defined time (1min minimum), or maybe extended by the player himself with a cursor (5min, 10min). The player can't stop this option before the end (it prevents from the cheated behavior. It doesn't supposed to be used to become invisible and disapearing for cheated)

I've played a lot of World of Warcraft (close to 12 years of it, yikes) on a PvP server. While WoW obviously doesn't allow for building structures to force a character in one spot, it does allow for the greatest of all pleasures in online gaming - player killing. Now, how WoW handles it is that if you kill an honorable target (within a certain level of you) it grants you Honor Points which can be used for whatever, and the victim respawns as an 'honorless' target, but killing someone lower level than you yields nothing.

I don't know how player levels (or if there are in fact, player levels) would factor into scaling or whatnot, so I'm kind of grasping in the dark here.

Does this game have a persistent character state while the user is offline? Like can I find them sleeping somewhere? I personally appreciate a very laissez-faire approach to player interaction. That doesn't mean I'm one to grief, but one of my sole interests in a game is making my own fun. Fun for me is character customization, PvP and ludicrousness.

Perhaps give a freshly spawned character 30 minutes to an hour (like I said, grasping in the dark here) of invulnerability to PvP so they have a fair shake at getting weapons and a feel for the game systems before they're mercilessly slaughtered and eaten by Kur Kal Ko, Who Was First to Kill.

In lieu of an invulnerability, maybe a starting zone? Getting everyone on an equal starting path is best. Equal opportunity allows for a fair start, but not equal results, which should be based off of skill and dedication.

Once logged off your character is gone, as in every other survival game

Really interesting ideas here!

Ah the only survival games I've really played are Rust, which has a persistent character in world even while logged off, and arguably EVE Online, which also has a character state while offline.

I think I mentioned this a bit in my first post. I can't remember where I posted it sorry.

Anyway my ideas would be to have a leveling system where the first couple of levels is strictly PVE. This gives players a chance to become established before getting randomly attacked as a new player without any idea of how to play the game or survive. This would give everyone an even start. So those new players would not be able to be attacked by another player. But they still have to battle against the environment and npc animals.

Safe zone huts or caves that are not pvp placed throughout the world can be helpful to aid in hiding holes for those who just don't feel like fighting that day. They can be minimally resourced places so it encourages the players to eventually move on but at least gives an option if a player is horribly outnumbered they can run here and try to wait out their opponent or wait for some backup to come online.

If barriers might be a problem. And the character in question doesn't have an axe in the inventory. Setting the computer system up in a way where it can detect a player that is trapped and upon re-logging would teleport that character outside of the structure. In reality most players should have an axe or weapon and be able to at least attack the structure and break it down themselves or simply move it out of the way?

I mainly play survival games, like ark, rust, conan, any many more ect the best way to prevent grieving by destroying everything the player owns when there offline is a simple no offline raiding mechanic this is where 10/15 mins after logging in and out of game that your base, boxes, character become invincible and inaccessible there still there in game but players cant loot you and cant destroy your base. by giving 10/15 mins either way it means you cant be ganked the moment your loading in and you cant combat log just avoid the pvp. as for blocking you in a cave have a disclosers saying that players found to be exploiting glitches and bugs will get a ban for a period of time first offence and a perma ban if there found doing it a second time this wont stop all players but would act as a deterrent, unfortunately as with all pvp games your gonna get trolls and griefers who's do it just because they find it funny and gen the only real way to get rid of them is for players to report them with evidence like vid of screenshot ect

I played alot of survival games like: Rust, ARK (not long); Dark and Light, Beast of Prey, Veritex, Citadel: Forged with fire, Life is Feudal, Savage Lands, Voodo (now Titans: Dawn of Tribes) and alot more.
If you want to hear my opinion/idea to that subject:

You have something like an Altar or similiar in your base/area which protect all you have but any player can destroy all but without an altar it goes faster and with an altar you need something like rams or similar (idk that much about the stone age) to destory walls better than wood otherwise it would take so much time. Also i would do some different doors like wood, stone-reinforced wood door and and and.., so a wood door is easier to desroy than a better door.
ALSO I would NOT take out the damage to other player until a war declare (it should always be able to fight/kill other players) because i saw alot of games dying when the pvp is non mainly.
To prevent offline raids i have a easy mechanic: TRAPS! Some different trap - the easier to build the easier to raid.
I would not take out offline raiding fully but harder with traps because its called a survivalgame and the main thing should be to survive all problems!

The other thing i would do is, sleeping bodys when offline because, when you log off to save yourself in a fight is shit and unfair in my opinion. Also when you get raided and log off you could save all the best things you have just that the raiders dont have it.

I know it comes a bit late and its just my opinion but when you look into Rust, popular as hell and one of the most played games in steam as a survivalgame. But what i miss in rust (3,2k hours in rust with 2 accounts) is something like a Clan/group/Guild so that you cant attack your own people.

*At last I also want to say that it should be possible to find his friends fast! In life is feudal: mmo you have almost every time 1-4h to find friends.

Best wishes,
Samradan