Illusion doesn't help you much with dragons, so conjuration would be a better choice. You'll always have a summon to do the job, though note that in essence you really don't need anything other than conjuration, so it may get boring for you again.
As for illusion on an assassin character, see http://www.gamesas.com/index.php?showtopic=1290251 for some discussion on that topic.
I tried some 1H+spell combinations on my assassin-ish character, but the damage seemed too low for when I needed to actually fight outside of stealth, so I just went with dual-wield, which costs only a few extra perks and works well (not always, but often). If you don't want archery, you can get the dragonrend shout which forces dragons to land.
I miss a shield. Shields are lovely in Skyrim.