Unarmored (Including Mage Armor) is the easiest way to raise the difficulty. The build I've done that Never got over powered was 2H assassin. No Backstab Multipliers, Armor, nor Magic (Beyond the occasional Vampire Dust.) You still have Sneak/Surprise, but failing that, you have to fight your way out, and you only ever get Double Damage. Invest in Health, lots of it.
I've also done a Support playthrough. Similar to the Pascifist, only instead of not killing, I basically followed Lydia through the main quest with Healing Hands (and others) to keep her off her knee. For role-playing, I've also done the Traveling Tinker, and Arms Dealer. Smithing, Enchanting, and Armor, but no weapon skills, and a bodyguard (In this case, Jenassa, or Uthgerd) to demonstrate my wares. Not unlike the Khajiit Caravan, you have a Merchant, and Warriors to fight off the bandits. Of course, you still have to go through dangerous places on the way, or into Dungeons to mine for Ores, so it's a good idea to hire some Protection.
I still agree with the Pascifist being the greatest challenge, especially if you Don't use Invisibility. Gets kind of dull, eventually, and severely limits the Quests you can take (Kill the Bandit Leader at...) but for sheer dificulty, that's the way to go. My previous suggestions are more somewhat challenging ideas to consider for fun play-throughs instead of the ultimate Challenge.
At least to me, getting bored isn't just the relative difficulty, that's easy to avoid by not doing all the things we do to make it easy. Crafting up to über power, taking advantage of Sneak, and Illusion, and Damage multipliers to never get hit, and 1hk. In addition to that, once we've done All the quests, and Achievements, killed every enemy, and become the most powerful mortal of the age multiple times, every subsequent playthrough is pretty much a re-run.
To counter this ennui, I role play. I'm not talking about adding Rules, like sleep, and food to make it more "Realistic" (In a game full of Magic, Dragons, Werewolves, and Gods/Demons that talk to people) but letting the Character decide whicch path to take, which choices, and how to approach every fight. This is beyond the Game Mechanics of the Build, but creating a holistic Charcter, with a background, motivations, goals, and Opinions.
Never traveling with a Dunmer, because she doesn't trust them. Avoiding confrontation, and face-to face combat, because she's a coward, or not using Magic because it's Evil. Having reasons for all these things that apply to the PC, instead of the Player looking through the screen deciding "This is too easy" and max/minning from the keyboard to make it more difficult. Try to get into the Character's head, see it through their eyes, feel their surprise, and fear for their life when that Draugr steps out of the casket just like it always does. At least for me, this doesn't make a whole new game, but more fun that just going through the same motions over again...