That doesn't work as spells have different ranges so that would only work with a specific spell charged up. Then again you might want it to only show it then. If you'd want it to work outside of that you'd need to have an effect on the ability icons.
Honestly, I may even prefer just having an indicator on how far your target is away from you. That would still require the most skill as you have to figure out yourself which spells work in that range etc. and it may be useful in other situations as well. I mean in reality you can also guess how far someone is away, that's what depth perception is for.