When you copy an at-will power or an encounter power: spend your highest level encounter or highest level daily power. You can spend an encounter add-on power or a daily add-on power instead.
So, why do you think that copying an at-will power should cost your highest level encounter power? That seems rather excessive. Again, when using this you 1) Have to wait for someone to actually
HIT with a power, 2) You often will not have the correct weapon to use the power or take full advantage of the power, and 3) Intelligence and Wisdom are your secondary stats meaning that your attack rolls with powers copied with Enemy Skill/Friendly Fire will automatically be a bit lower than normal.
When you copy a daily power: spend your highest level daily power. You can spend a daily add-on power instead.
Just to clarify there are currently no daily add-on powers throughout the first 19 levels, so therefor I don't see any cropping up in the last 10.
Finally, you are talking about copying higher level powers than normal with Enemy Skill/Friendly Fire, but if I'm concerned about balance shouldn't I want you to only be able to copy powers that are no more than the same level as yourself? I may be misunderstanding what you are trying to say.
I have an idea to further add some balance to Enemy Skill/Friendly Fire:
What if I just throw in a limitation that you can only use it to copy at-will powers twice each encounter, encounter powers once each encounter, and daily powers once each day. That way, if you copy an at-will it doesn't cost you anything but the action, but you can only do it twice per encounter, and if you copy an encounter power it costs your highest level encounter power and you have to wait until your the next encounter to copy an encounter power again, and once per day you can spend your highest daily to copy a daily power. Seem reasonable to you? Seems so to me.
Finally, since I'm done with Paragon Tier powers, I'd like to work on the Paragon Paths before I move on to the Epic Tier powers. I really only have vague concepts for two paths right now - One of them more Arcane Focused, the other more Martial focused. The Arcane one I want to be Intelligence based, and encouraged to wear cloth armor in some way. The martial one will still have a taste of arcane, but should come out with a defender angle probably. Any ideas to further flesh out the concepts? Any ideas for other Paragon Paths?