Okay so I'm thinking of

Race: Imperial

Birthsign: The Lady

Specialization : Combat

Favored Attributes: Strength and Endurance

Majors skill: Alteration,? Armorer, Blade, Block, Destruction, Heavy Armour and Restoration

Should I replace Destruction with Conjuration or Mysticism? Or should I just keep Destruction?

I personally would drop Armorer and Destruction and pick Illusion and Conjuration as replacements due to the Turn Undead from Conjuration and the Rally/Demoralise from Illusion. EDIT Your really playing a spellsword without Illusion, Armorer is a great way to build Endurance, but I would use it as a Minor Skill to try and get a +5 to my endurance on early level increases.

I'd Say:

Ritual, it gives you a lesser power turn undead spell IIRC

Heavy Armor
Blade (for your Claymore)
Athletics (To run down those Necromancers)
Restoration (To heal and Bless yourself and your allies)
Alteration (To Augment yourself and your Allies)
Destruction ( I'd get damage health on touch and treat it like a 'Smite Evil' spell. Limit it's use.)
Mysticism (Absorb/reflect spell for fighting those evil conjurers)

Okay so I should Replace Destruction with illusion?

no, i wouldn't destruction levels kinda slow, and illusion levels pretty quickly. Just go and get a starlight spell and cast it over and over again

My 2 cents:

You probably don't want more than one skill for any attribute in your majors, or you have less control over levelling. So pick only one from Armorer, Block and Heavy, as they all are Endurance skills. Even when playing an almost pure mage you can pop on a heavy suit, summon up a skeleton and get it to whack at you when you need more endurance, but with all three as majors, that will level you up before you reach +5 if you increased any other major. As a melee character, you'll just be levelling too fast.

My preference would be to have Block as the major, as you are in total control of using it. Heavy armor increases are controlled by those hitting you, and armorer will go up if you mend your own gear, which you'll want to do rather than spend gold.