There have also been rumors that Vader were somehow returning, right?
While they may just be spinning those rumors off of some of the leaked concept art, just the fact that they are using vader's remains/helmet in the film AT ALL is still a potentially big clue.
Let's say Plagueis is the villain. And either him or his dark side trained goons are the ones who are looking over vader's remains in the leaked art.
Why would they waste any time at all showing this in the film if it weren't of some importance to the plot?
Remember how pivotal Plagueis is to Anakin's downfall... though, in an indirect way. For Vader and Plagues to meet or somehow team up could be interesting if done right. And that really is the key... if they were to somehow bring back vader, whatever form that may be, they need to do it in a way where it doesn't come off as eyeroll inducing.
I don't think it's impossible.
Hang with me here, because I think clone stories are very easy to fuck up... but if they cloned vader, and he were young again, like say as young as he was in Episode III, and he knows through Palpatine that Plagueis has mastered the power to cheat death, it's possible that Plagueis could somehow use the potential for reviving Padme to gain Vader's trust or help.
I don't know if that could work. But now that we have some information that both are potentially in the film, it's a line of thought that is hard to overlook...