Diablo 3 already makes the tons and tons of pets from demonhunters and witch doctor are nearly invisible to other players.
In Legion with another melee class and demo warlocks and BM hunters adding more melee pets than ever it would be really nice to have the same feature (make it optional for people who enjoy the clutter) so you only see your own character spells and maybe even make other melee players turn transcluent?
If you don't know Diablo3 in Multiplayer - check the witch doctor in the lower left of this image
He is surrounded by his pets - but they are opaque to me since we are in multiplayer.
Something like that would be nice in Legion.
For long enough melees have to stand in a clusterfuck of effects that are not cool, helpful or interesting: