I made a quick 5-hours playthrough on this build, and having lots of fun.. He's an Redguard Assassin, born under the Thief, Speed and Agility..

On the first playthrough I had: Blade, Block, Light Armor, Marksman, Conjuration, Sneak and Illusion... The problem is - I never used Illusion... This build is not build up on a lot of magic, and having Conjuration as a back-up was fine for me..

Soooooooo - what skill should replace Illusion? I'm already using Alchemy, but thats fine as a minor skill. I was thinking about armourer - but is it useful at all? It could be good in roleplay though.. a lonely assassin, who's capable on handling everything by himself.. repairing his stuff, and making potions...

But VERY keen on listening on other contributions

After reading the first part of your post, I was gonna suggest armorer. After a very long fight, it's nice being able to repair all your equipment without going back too far.

Or you could take Acrobatics and go all dodge-tastic after reaching 50.

Esspecially considering assassin armor is enchanted so a 50+ armor skill would be pretty useful for you.