Hmm... you're right. Feather and Agility have been 100% confirmed. Celerity, Healing, Fire, and Ice have not been confirmed. I thought fire and ice wouldn't be in because they are exactly the type of thing Dishonored is trying to stay away from.
Why not? I can imagine interesting uses.
Freezing water to cross a lake. Freeze a Tall Boy's Legs. Freeze a Tall Boy's Arrow.
Using fire to light candles. Light tapestries on fire to find secret pathways. Using fire as a distraction. Using fire a DX like Torch.
The the generic uses which I think you're implying (fire ball, Ice Blast etc) should still have their place, but they should be costly; kind of like how gadgets are costly for the technological path player.
Up to now I thought windblast was the only elemental magic to the game. It's a cool feeling knowing you can play as an 'Elementalist Assassin'.