I see a lot of people running minion-master, but I think minions are more something that will draw agro from a player and nothing more.
So, first thing I would change is powering minions by life force. In exchange for more powerful and useful minions, you have to pay with your life force. This is something like Revenant's energy based skills. Each minion would have it's cost with X life force per second.
Minion's skills (these after summon) would turn into commands, that would cause your minion to destroy itself and do damage/inflict conditions.
So... lets explain it.
Blood Fiend currently heals for around 1k hp per 3 seconds. It's OK, but I think doing a combo with Bone Fiend (which inflict bleeding) would make it more powerful. Necro should have better heals. Healing for 3-4h if you have 18-20k hp feels like nothing.
Flesh Golem has strong CC, but with these changes it would have 2x CC - taunt and knockback.
Bone minions needs an effect after they explode, so creating a combo field would boost e.g. Flesh Wurm's and Bone Fiend's attacks (combo finisher: projectile).
Shadow Fiend should torment people (because of haunting), not beating them to death. It's a shadow.
After you run out of life force, your minions just die (no effects).
Getting out of combat would change "command" skills into "hide your minion".
I don't like sand. It's coarse, and rough, and irritating, and it gets everywhere.