...without being an utter mediocrity?

I've had an idea for a Dark Elf character that would require concentration in:
? Destruction (fire)
? Necromancy
? heavy armor
? either one-handed or two-handed
? smithing
? enchanting

My previous character was a Nord Warrior focusing in one-handed, shield, heavy armor, and smithing, so only four skills mostly. This one I basically want to look like he comes out of a Frazetta cover, an cover of Heavy Metal magazine, or maybe some metal album. A Fire-Wizard, Necromancer, and "Evil Warrior In Spiky Armor He Forged Himself From The Tears Of The Innocent", so yeah - basically Sauron I suppose.

But six skills as actual fields to focus on? Is that really feasible or am I dooming myself to being underpowered and a jack-of-all-trades-master-of-none?

Your potential character seems like a "Dark Battlemage" to me. I never take two crafting skills as majors as I find it's really easy to become over powered. Maybe drop smithing and enchant armour you find or buy would be my suggestion, otherwise it looks like a pretty good build to me.

I plan on using enough mods to where hopefully the game will definitely be challenging. I was just worried that six majors would leave my skill levels lagging far enough behind my character level that I might become under-powered.

I typically assign three "Majors" per character. I restrict perk point to those three skills as well. I've never had a problem leveling most characters on just three skills.

My last character, a warrior, had Light Armor, 2-Hand and Smithing as "Majors." I played on Adept and used Deadly Dragons, Deadly Monsters, Skyrim Redone, Organized Bandits and a few other mods to increase difficulty. I used Elys' Uncapper mod to slow down skills I couldn't avoid using, like Speech, to almost nothing.

I find that three skills is usually enough for the majority of my characters. But everybody plays a little differently - and each character may be different also - so it's up to you to decide what feels right for you.

Well, those six definitely seem important to my character concept. I was just afraid of six skills pushing my character level too far above the skill levels for me to keep up with the bad guys in the field.

I guess it shouldn't be an issue; there's generally a way to make it work.

I usually put perks into 3-4 skills, and take another 2-3 as minor skills that never get perked, and generally never make it past about 35 or so.

More often than not, I never make it past level 50 before my character has told his story and gets retired. Anything beyond that, and I find they start getting a bit too overpowered. Then it ceases to be fun, and I find myself starting a new character before long.

Think about how many perks you will need. In order to get to level 50 you need to get 6 skills to 100.

One thing to recall is that the more skills you keep up with, the more challenging the game becomes. Your character level will increase, making enemies level to that aspect. However, your main skills may lag somewhat so that you don't have all the "nifty perks" as fast as usual. It is not unlike the players who found the "smithing iron daggers" glitch and leveled up smithing to 100 right out of the gate only to find it hard to deal with enemies thrown at them, with no combat skills leveled.

I mention this because the last character I played had a great smattering of skills that she kept up with and the enemies did not "feel easy" until the very end. I actually got her to level 57 before retiring her.

Focusing on a single skill is vastly underrated. I usually start mages off focusing only on destruction and use nothing else. By level 8-10 I am rotfl stomping everything with dual cast impact fireballs.