Answer to primary needs [eat / drink / sleep on a regular basis] - check
Don't carry more than 50lbs - check
Set the timescale to 4 so 15 minutes IRL equal to 1 hour in game - check
Develop only the skills that suit your character - check
Don't use fast travel [use carriage instead] - check

Anything else?

can use pcers add degradation scales/times to your skills if not used regularly enough?

limited potion use and effects over time.

Don't use potions, make the game too easy.

Dead is dead, only for the most hardcoe players.

Don't level up, perks make game too easy.

Carry 50 pounds? Where did you get your backpack? I never got one, so I carry what I realistically can carry with just some pockets and my hands.

Roam the world as if you don't know where everything is.

Don't use map. Don't use compass.

Don't do quests that you already know about. Learn about them first.

That is mine for right now.

Thanks for the ideas. I got already used to not use neither the map nor the compass in my "casual" playthrough. I'm afraid DiD is not my speed but I'll put the other tips to good use.

turn the hud opacity completely down.

In my more hardcoe game, I prefer to develop the "useless" skills, like lockpicking, speech and things like that.

I have no idea how easy or hard it would be to add this... but lockpicking, opening chests, looting stuff and being in your inventory shouldn't pause the game.

Get rid of enemy health bars if you can.

You`ll have to judge their state. Not as hard as it looks, but be ready to run if they knock your health to 25% and still look strong.

It would be a great addition but I'm afraid that if it works like in Oblivion, it's hardcoded in the game and no one can do anything about that. We'll see when the CK is out.