Feel free to take my ideas and use them.

I think King Sombra has the potential be a villain as bad Sauron from The Lord Of The Rings.

I think of King Sombra having a dark army of Ponies turned evil under his control.

Personally, I think the Diamond Dogs are more then perfect as his minions. Think about it!

1. They both take inspiration from Lord of the Rings, or so I'm told of the Diamond Dogs.

2 King Sombra is an evil overlord in need of minions, and the diamond dogs are like minions in need of an evil overlord.

3. They share interests in crystals and slavery.

4. The Diamond Dogs are dogs, so their loyalty should be a huge motivator for their master.

I even have King Sombra create their whole race for this very purpose as one of my headcannons.