I'm not sure if you're still doing this, but...
I could use a spell for my map. It turns a unit ethereal and slows their movement for about 30 seconds. If they aren't dispelled by the end of the 30 seconds, they turn into a minion of the caster. Here's what I was thinking:
Level 1 - Slows movement by 20%, target has 30 seconds to be dispelled.
Level 2 - Slows movement by 30%, target has 25 seconds to be dispelled.
Level 3 - Slows movement by 40%, target has 20 seconds to be dispelled.