With everyone else scrambling for villain roles, maybe Gary Oldman can finally be the hero?
The thing with the Joker/Venom/Chigurh trend isn't that they got the most screentime or top billing, it's that the directors are trying to emphasize that the villains play a role that's as important as the hero's. In fact, I don't think any of them got top billing or the most screentime in the movie. Chigurh was the first villain in recent films to get an Oscar, but he's by no means the first, and pretty much everyone involved in that movie gave a damn good performance.
The reason for Venom and Magneto? Both have been in a lot of comics and have cool superpowers with which to create special effects for. As for Young Tom Riddle, it's merely timing that it's happening now. In the 7th book, he gets much less screentime; it's focused on the journey of Harry, Ron and Hermione, and some of Dumbledore's backstory/general wizarding history. You could say he's barely going to be featured until somewhere in the second movie when it picks up.
To me there doesn't seem to be much of an upswing in villainy, it's just timing of movie releases. I don't even know when the Venom and Magneto movies are coming out, anyway, the whole thing will probably have died down by then.