There are a number of immersion and roleplay principles I adhere to, some diligently, some casually. Most are common stuff: no fast-travel, travel at a walking pace (both on foot and mounted) except for brief spurts of sprinting/galloping, not riding my mount in inappropriate terrain, sleeping at night or catching naps during the day when commitments break normal sleep routine, limited "waiting", refusing quests that don't fit my avatar's allotted "role" in Cyrodiil. That sort of thing. But I never think of these as "hard core", maybe because the bulk of them come naturally to me and are no burden.

-Decrepit-

None. I use ZERO hardcoe rules.

I don't need them to feel immersed in the game.

Incidentally, the word "immersion" has officially made it onto my "Words I [censored] Hate" list.

With my at this moment,
Not using any melee weapons. Only us bow and offensive spells for attack.
Only use Mortar & Pestle only for potions and poisons and suffer if or any negative side effects from drinking potions.
Sleep.
Don't use any chapels to cure sickness and loose of Attributes. Only 'live on the lam'

edit: Not visiting Imperial City until I done with Mages recommendation quests