If you want to mix in Destruction, I would say focus on the Rune spells - they're expert level and essentially just traps. If you can silent cast (Illusion perk) one in the path enemies will take to get to you, it'd be a nice defense. For reference, know that you can't silently disarm them after you cast them. You'll need to either take the hit, throw something onto it (I think that works) or cast the same spell type onto it. Like casting Flames onto a Fire Ruin will set it off, but it won't set off a Frost Ruin.
Conjuration can be cool, especially if you go with Necromancy. Kill a target, then enthrall it to fight his friends. Otherwise, it works well simply to tank should you be seen.
In my opinion, however, you're leaving assassin behind and going more for a stealth mage.