I've thought about this quite a lot myself and heres an idea, you can add abilities to units but if you add an aura it takes up a slot and if you try and max out the boxes it will bump something off most likely (Unless you can figure a way to have the aura bumped off and it still works).
I have found something out that works rather nicely, probably can't be used in a practical sense but say for example you GIVE a unit the ability of Item Life Regeneration, no icon shows up...but the effect of the life regeneration still works. Create a spell that is a 0 second cooldown no mana cost...this will be the trigger. Create a variable that is initialized to 0. Now if you know anything about spells do the event cast condition is the trigger action is set variable to 1. Run another trigger now that checks units within the range of our unit with the aura, if the variable is set to 1, add the ability to the units within the range, otherwise remove the ability from all other units.
I've done this myself in my map, its not pretty, it does work and I'm attempting to optimize it.