As title states i think they should make so you could use other demons other than your felguard when using demonic strenght, it doesnt say “demonic felguard strenght” so would couldnt it empower the other 4 demons we standardly have.
With the new changes coming to demo in patch 10.2 bilescourge bombers have been buffed to a point in which it make be taken over demonic strenght, making the seem more and more that blizzard wants us to use abilites related to our felguard less and less.
My suggestion is rather simple and just making version of that spell and perhaps also the right side of T3/2 talents also effect your other summonable demons.
This could vary from your imp just cleaving with firebolt and perhaps making single magic be able to aoe dispell or dispell everything.
They could even make so if your dispell something its applied to your current target.