[FatherCrixius]

Don't be a dime a dozen 'event only antagonist' that only has stakes in isolated areas, it's cheap and next to never has any narrative weight. Opt for more 'petty' antagonism, establish yourself as a staying 'threat' to player characters.

Avoid making yourself manic or overly sympathetic, remember that every good villain rationalizes themself as doing the right thing, for one reason or another.

Know that few people are going to offer you resistance or even return your antagonization, and that it will be difficult to get a reaction - this is something that made me avoid playing an antagonist myself. You'll either be goon-ganked by over-reactive protags or receive such a lukewarm reaction that it would feel unjustified, even for a villain, to continue antagonising the vacant.

There's probably more but i'm sure other people in the thread will pick up on.