To be really honest, I kind of miss the mustache-twirling "throwback" villains of yesteryear's comics and cartoons - the type of villain that is evil, knows it, and most of all has fun with it. I never really see this type of character in video games. I think that in relatively light games where you don't spend a lot of time getting to know the villain, the Dick Dastardly-type could be really enjoyable.
Personally? I kind of like when the expectations get inverted. A "wait, were we the badguys the whole time?" type thing.
Don't get me wrong, I hate playing as the villains in a game. But, a game where I didn't know I was playing as the villain and it's revealed that what I was doing was horrible and terrible and the "villain" was actually the hero trying to stop me...
I'd like to see that a lot more. I find it more interesting and fun. It really messes with my expectations and with traditional morality.
Probably why I do it in D&D campaigns a lot. I like to make my players into the villains by just providing the gameplay they instinctively seek (namely, being murder hobos).
I like bosses that don't actually turn out to be villains at all, and while their actions might be less-than-heroic, they are more interested in their own agenda that doesn't involve you but are fine with teaming up with the heroes and stopping whatever larger-scale threat that pops up. This is especially so when they stick with you in the long run and become an actual friend (Whether they want to admit it or not!).
There are also those ones who just want to fight you without any malicious intent, facing you in battle is their idea of fun times! A Prankster who can get in your way, but is generally harmless. They can also be sizing you up for later encounters, seeing what you're capable of so they can assist you accordingly later.
Oof, this "side project" of mine is starting to get as complex as a VS plugin, at least as far as plugin parameters are concerned. I think I now understand why a plugin like this doesn't already exist(that I know of). Learning a lot while trying to make it work though!