Darth Vader so embodied the Black Knight archetype, with added gadgets and Force powers, that he moved from interesting henchman to central character, and all the other villains in Star Wars went with other styles of supervillainy. The Emperor was originally going to be a distant and isolated figurehead, but became a full-on Evil Wizard.
When the prequels rolled around, a different kind of Darth was needed, so we got Darth Maul as Ninja Satan. Count Dooku was Sir Christopher Lee With A Lightsaber. General Grievous was a four-armed robo-skeleton.
Kylo Ren had to follow Darth Vader in-universe, so I think making him a Vader Fanboy was a stroke of genius.
I borrowed from outside sources for my last Star Wars villain, taking the Kingpin from Daredevil as the basis for a leading Imperial intelligence officer.
What would a Star Wars version of Ultron be like? Or Loki? Or Bane?