Nehtack has been around since 1987, so bethesda have no reason not to blatantly plagiarise it. Here are some things from nethack which would have made Skyrim better.
1: Interesting enemy attacks
In Skyrim, enemies hit you until you die. Your job is to hit them until they die. You can hit each other with spells, swords and arrows, but it’s always the same, you chip away at their HP while they chip away at yours. You use the same strategy for every enemy, even ones you have never encountered before.
In nethack, enemies will mess with you in all kinds of ways. Nymphs will steal some of your items and then teleport a short distance away. Puddings will divide in two if you hit them with an iron weapon. Krakens will try to drown you. Succubi / incubi will attempt to seduce you and take off your clothes, leaving you naked and armor less until you put them back on. If you hit a rust monster with an iron weapon, it will corrode. Ravens will peck at your eyes, blinding you. Some monsters have passive attacks and can only harm you if you choose to hit them.
All this means each fight must be approached differently. This is especially the case when dealing with many enemies at once. Do I blindfold myself to negate the umber hulk’s stunning gaze attack, or do I deal with the nymph first (after all, she could steal my blindfold)? Do I take off all my armor and sheath my weapon so that the nurse’s attacks will increase my maximum HP instead of hurting me, or will a team of soldier ants show up and catch me with my pants down?
It would also make other aspects of Skyrim more interesting. What’s the point of stealth? All it gets you is a damage bonus to your first attack. If each enemy needed to be handled differently however, getting the skinny on enemy types before the biffo starts would make a huge difference. Also spell effects like “detect life” would be much more useful if you were temporarily blinded, using it you could still locate and attack the enemy.
There are gods in Tamriel. If you stop at their shrines you get a blessing. Some of your stats go up temporarily.
In nethack, you can make sacrifices to your god, who might give you pressies like really good swords and armor in return. If things are going badly and it looks like you are going to die, you can pray and they will help you out. Conversely, if you do something to anger your god (like killing one of their priests so you can take his loot), they will send minions and/or lightning bolts from the heavens after you. Nethack is full of these risk/reward choices that make things interesting. The only interesting choices in Skyrim are whether to go for spells or swords.
3: More ways to die
In Skyrim you die when your HP reaches zero. In nethack you can be killed from food poisoning, disease, starvation, sexual overexertion, being petrified (turned to stone), being drowned by a sea monster, having your brain sucked, or from accidentally magically genociding all creatures of your own race.
4: More enemies at once
In Skyrim you might face 10 enemies at once, a few times in the game. In nethack you can be facing fifty. Sure that would be hard to do in Skyrim without grinding your PC/console to a halt, but perhaps a swarm of small enemies (e.g. low polygon rats) wouldn’t look too bad, and could change things up.
5: The dev team thinks of everything
In Skyrim, you can turn into a vampire or a werewolf. In nethack, you can polymorph into any enemy in the game. If you turn into a Dragon and your character is female, you can lay eggs that will hatch into baby dragons who will fight with you. They will eventually “level up” into adult dragons. If your character is male, don’t worry, magical sex changes are also possible.
In skyrim, you can wield swords, maces, bows and spells. In nethack you can bash enemies with a dead cockatrice, which will turn them into stone. Just be careful you don’t fall down the stairs while carrying one, else you will touch it and be petrified yourself.