• Illusion for Quiet Casting perk and Invisibility spell.

Quiet Casting makes casting silence therefor works well with re-casting Invisibility.

• Conjuration for Mystic Binding perk and Bound Bow spell.

Mystic Binding perk increase damage done with Bound Bow.

• Destruction for Rune Master perk, Augmented Frost\Fire perk and Fire\Frost Traps spells.

Rune Master perk will help you cast it from far away in the shadows (hopefully Quiet Casting will impact this somehow)
Also take Augmented Frost perk or Augmented Flames perk for increased damage with the Traps.

• Sneak for Stealth perk and Deadly Aim perk.

This is to increase the damage with Bound Bow by 3x while in stealth.

• Enchanting perhaps to make the enchants on your armor more benefictal. Better mana regeneration.

I see this as the best Stealth-Mage Assassin build I could think of, that plays out as you are always alone against many enemies in dungeons or perhaps on an assassination quest.
Also this build will let you avoid dealing with Alchemy & Poisons as an Assassin that is my idea of moving more forward of being a Mage Assassin rather then an Illusionist Assassin, also adding the whole Destruction Trap part will also "strenghten" the idea that your a Mage Assassin and not just a Illusionist Assassin or ordinary one.

But what do you think?

Choose fire vice frost so illusion perk "aspect of terror" boosts fire spells by 10 points.

If you are only going to use Illusion for Invisibility, then you are wasting perks in the tree. Use alchemy to make invisibility potions. You'll save perks because you won't need to spend *any* in the alchemy tree to make decent potions. You also won't have to work your Illusion up to 70.

Seriously. Don't waste your time on the Illusion tree if you're doing it for one or two spells. On the other hand, if you plan to develop it and use the calm/frenzy/fear spells, you may not need much else for your build (Illusion is *very* powerful).

I've made a female Atmer as my second character like this, though I was originally considering a Dunmer (As I thought a female would be further removed from my main male Nord warrior I thought a yellow skinned & raven haired Altmer would be more appealing to look at).

I've gone for stealth and illusion (invisibility etc) and stay light footed for sneak, but have found myself tending to use destruction a lot for combat and I've stayed away from weapons which wasn't my original plan.

As in all character builds in Skyrim... just play the game. Your build will become defined by your actions before long. If you envision yourself as a Sealth-mage-assassin, you'll become one.