As DLC seems to far away I take on a pre DLC character and I like to think about ilusion assasins with bow - cutthroat is to mainstream

Thats pretty much the character build I am playing right now and it has been my favorite so far. Although he does cut throats as well, lol. I used to think Illusion magic was pretty useless but its by far my favorite school now.

you can also have alchemy, to improve illusion and add more damage to your bow.

I'm currently playing a pure illusionist/sneak/alchemist, NO direct damage. It's a lot of fun.

I am always leary about using Illusion and a bow as you can do the vast majority of everything with the bow and not need to use your illusion. But, there are situations where it is downright impossible to sneak and crowd control is useful.

I have a fighter thief that uses illusion and no bow, and it is great fun casting Calm on someone than strolling behind them, entering sneak, then using Assassin's Blade. You get the cutthroatscene about 100% of the time.

You might consider an altmer as they are awfully good Illusionists.