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Try a weapon with a damaging affect that makes it so that the closer enemies are to the "center" of the effect, the more damage they take.

One example of doing this is enchanting a bow with:

10 fire damage; and
10 fire damage within a 10 radius; and
10 fire damage within a 30 radius; and
10 fire damage within a 50 radius with a 10 second duration

i don't know if all that would be able to fit on a weapon, even with a grand soul gem, but you could try something similar to it.. The effect would do a little damage to enemies that are further away from the blast, but when the enemies are closer they will get hit with more effects at once, doing more damage, plus the "duration" on the long range effect will set enemies "on fire" for a while, doing more damage over time.

Does that sound like a good idea?
(by the way, I just thought of this when i read your post)