Criticals
Here's the thing. Nord brutality inspired, in TES:V we could have melee Critical Hits that make combat feel medieval, that cause broken skulls and bones, or simple bleedings over time. As rare as criticals are, they could cause only a little or no more damage but cause instead any of these effects which are in turn the "more damage" they could realistically cause.
And no, the effects are not brutal, they affect you or the enemy only somewhat, so this doesn't necessarily mean EVERY time you receive a critical injury questing is over looking to heal it. You can keep going if you can't heal it yourself, you'll just have it a bit harder.
And you know, you could deal this effects to enemies too so it's not really about making it a harder game: It's about making combat feel more engaging and realistic at the same time.
All of them should have a screen shock effect when they happen and maybe a scream and could all cap your max health a bit. You can't be full health with your skull cracked or a suddenly more flexible extremity Let me explain some more.
Crippling
Now for those crippling effects. Besides Fallout and Beyond Fallout, this is what Skyrim could approach:
A critical to the head causes a broken skulls that leaves you numbed and severely head-ached. This means your intelligence and speechcraft lower a little.
A critical anywhere else causes a broken bone which would affect all your weapon stats if the game features no localized damage, but if the game recognizes between arms vs legs it could affect weapon stats vs speed/stamina/agility.
Torso criticals could cause a whole but slighter combination of all that. That simple.
And of course, these injuries would be signaled in icons in the Stats interface telling what the effects are, no mysteries.
I don't mention strength because that would many times cause an instant overburden and leave you standing still in the middle of a fight, and while kind of logical it's a bummer and there's no chance they would implement that.
Magic
I don't think magic causes criticals. One attack doesn't cause more damage than exactly the same attack again of the same magic (unless it's a fireball to your nuts ) If anything, as a way of criticals they should have a chance to: Burn sticks for a couple secs, Freeze freezes you in place for a couple secs (like Dark Messiah), and Lightning stuns you in shock for a couple secs standing or to the floor.
They could also have a chance to alternatively cause burns (Fire, Freeze and Lightning the three cause burns if you didn't know) that simply cause some more damage and could stop health regeneration.
Healing
This would also require some special medicines or healing mechanisms. There could be healers in towns to treat you for a fee and specific medical resources in like a single "critical medkit" (tourniquets, splints, ointments to heal your burns and bleedings) to carry with you (besides healing potions). Or "Morphological Restoration" healing spells for mages for critical damages too! (wow, i even came up with a fancy name and everything! lol) Or the three separate if you want it hardcoe.
As a lighter alternative which i don't like but some may want, you could use regular healing potions specifically on these injuries.
Alchemy would also allow you to create ointments for burns and bleedings.
And after all, for the player, they could increase into Severe status if they are left unattended for more than 2 in-game days. Of course i hope you love yourselves a little bit more than that! And if some of you would want to die out of them, let it be for broken skulls only or maybe none at all.
How do you find these mechanisms? I think it adds A LOT to combat and makes it feel so much more engaging and deeper almost as a whole new experience, causing all that damage and hopefully receiving much less than that too!
To sum it up: This is just a way to make only Critical Hits better and more exciting, adds some slight effects for you or your enemies and improves on magic criticals, and you can heal them with special medkits, spells, maybe healing potions, heal burns with Alchemy and Healers in towns. Let it fly in the replies and thanks for reading.
Disclaimer: I took the idea of having critical injuries from other D&D based games like Neverwinter Nights, Dragon Age and Drakensang so as you see it's nothing too hardcoe, and then fleshed it to my own imagination including the critical effects, magic extra effects and healing options.