The character I've found most viable on Legendary is a simple Destruction / Restoration / Alteration mage with only those skills and a bit of One-Handed. If you use stealth attacks as a primary form of offense you will die a lot, because it will rarely one-shot the opponent and then you have to fight them up close.
Conjuration can help, although it's possible to survive without if you use Shouts skillfully. Also, if you get Whirlwind Cloak you will not die, because melee characters (the usual hard hitters) won't even be able to get close to you.
Sneak is your best friend. Also spells that do not cause direct damage, like Illusion and Conjuration. Find spiders in Dragonborn DLC - Cloaked ones will destroy everything if you make them on high levels (their level is equal to yours). Usually one crafting skill is enough to make a character viable, at least Alchemy is.
Depends,
Hardcore Skyrim session: LEGENDARY TO THE MAX
Feeling like roleplaying: usually Adept but sometimes Expert
Faceroll relaxing time: Novice or Apprentice
I usually play on Adept cuz its kinda relaxing while being challenging at the same time. I only use Legendary for specific characters. Sword and Board works best.
Ive played my shadowcaster and brute on legendary, but only after about lvl 45 or so. I think Legendary straight from Helgen is too cruel a punishment to inflict on oneself.
I prefer to balance my builds for master. I feel that in order to truly determine your build's viability you got to play on at least expert.
I agree fights take longer. But your potions run out faster too. Then you're left with your most viable options for a Legendary gameplay. Skills that you don't normally use will suddenly become very useful. Money will become very important (unless you use the console for cheats).
Lots of people say "I stopped because I was bored" but change comes out of boredom. What's more rewarding, playing a difficult character which you'll turn into a very interesting gameplay (if you can and want to) OR drop your character and start another one (in doing so, going through the same stages you've been through tens of times before, hence getting even more bored) character which will be slightly overpowering because you were bored with not being able to kill your opponents faster...
Then there's your time investment. If you only have 20 mins a day to play, probably in 20 mins you need to do 10 quests. Personally I go for quality over quantity. It annoys the hell out of me to spend 3 days IRL in the same dungeon, but the feeling of accomplishment will be considerably greater. And in 3 days time, the gameplay I would be forced to adopt will change the build's overall gameplay (because I would not spend even more time, getting out of the dungeon to buy items which I would need to finish the dungeon off faster or more easily).
Yep, it's just a question of changing mechanics. Being a Breton and having a special Standing Stone does the same as a number of potions. Getting hit with physical attacks can be avoided or mitigated in various ways. Doing more damage can be modified by various means too. It's just a matter of choosing what makes more sense/ applies more to your preferred gameplay.
In the end, you will stick to a couple of mechanics which fit better with RP and personal preferences. These will always win versus creating an overpowering character which will get you bored with the gameplay all the same.
From my experience creating an overpowered character on Legendary is quite a hard thing to do if you limit your crafting skills to one. The only thing that I really felt overpowered was Ignite with Aspect of Terror, Augmented Flames 2/2 and Fortify Destruction 80%. Still, without the potion there were enemies quite able to kill you, like Dwarven Ballista's
You could just keep on you a stack of standard potions: Fortify Two-Handed, Fortify Block, Fortify Heavy-Armor. Apothecaries will start better and better versions of each potion effect in increments of 10 levels (lvl 1 < 10 < 20 < 30 < 40). During your first 10 levels you can simply run around making money, training your skills as you go. I never get why people say that playing a high difficulty is so hard, in early levels.
Ah, you refer to defensive "overpoweredness" My thoughts were about how to kill enemies as easy as on adept with only one crafting skill. Still, I find these dwarven machines and draugr death overlords cutting through capped defence even with capped AR enough to make you feel worried