Hi, I'm new to this website and saw your post so I thought just to relay my suggestion for a Ninjamage character.
First off, I agree, screw Mystiscism. I never use Mysticism, you're likely to find a ring or necklace with a Constant effect. (e.g. Ring of Detect Life - Detect Life for 25ft)
I'll admit that I haven't been playing this game for long, about 4 weeks now, but it's the only game I've been playing since I first got it.
Okay, here we go.
In my opinion, I don't think it matters what race you are. I recommend the Breton. My friend suggests the Dark Elf.
Breton - They have a +50 to their Magicka.
They have Strength against most forms of Magicka.
Dark Elf - They have a 75% Strength against Fire.
They are very effective with the Blade and Bow.
But it's as I said, I don't think it matters.
Birthsign - The Mage. For it's +50 to Magicka, with no "bad" side effects at all.
Class - Custom
Specialization - Magic
Attributes - Agility and Endurance. For Marksman, Sneak and Health.
(You can choose any other Attribute you want, but I strongly recommend Agility for sure.)
Major Skills are as follows:
Athletics - The higher your skill level in Athletics, the faster you will recover your fatigue.
Alteration - For defensive effects and burdening your opponents. (Most burdening spells are touch, so you must sneak)
Blade - For close-range combat, and believe me, there will be close-range combat. Athletics will help to recover your fatigue faster from attacking your opponents.
Block - For defense against attackers. After a skill level of 25 you will no longer lose fatigue from blocking an opponents' attack. Blocking effects can be improved with an Alteration spell, such as Protect or Defend.
Marksman - For long-range Sneak attacks, improving this skill will give you a + bonus to you Agility Attribute upon level increase.
Restoration - For Restoring attributes, (such as Agility) Fortifying Skills, (such as sneak) recovering Health and Fatigue, and curing disease, poison, and paralysis. Note: Porphyric Hemophilia is a vampiric disease and you will have 3 days to rid yourself of it before you become a vampire yourself. Cure Disease will be a godsend. (You can acquire this spell from the Chapel of Stendarr in Chorrol, but requires a restoration skill of 50.)
Sneak - The higher your skill level in Sneak, the less likely you will be detected. Also higher skill levels such as Journeyman will allow significantly higher critical damage with your Bow or Blade while sneaking.
There you go man. You can give this a try if you like. In fact, I may try it myself. Have fun and Good Luck.
- Deej.
PS, I apologize for not adding Light Armour or Alchemy, but you can still focus on them as a Minor Skill, the skills will still improve and add to your Attribute bonus upon every Level increase.

(Intelligence and Speed) You can also create poisonous potions in which to dip your arrows in that are very deadly when used in combat.

I have yet to fully understand potion making, but if you have a good grasp on it, then it should be no problem to quickly increase skill levels for it.
PPS, Also, I welcome any feedback regarding this post.
PPPS, All skills provided as major are, to the best of my ability able to increase your overall level gradually.
Edit: Apr 04, 11