Critical to the success of Star Wars 7, and the resurrection of the franchise back to something of real substance, and back to something of real legacy (think the various Marvel and DC comics), is getting the main villain right. If there suddenly appears another Sith Master, out of nowhere and against the Rule of Two, then this is total creative folly.
The only way to get the villain right is to connect him to the previous trilogies, just as the heroes no doubt will be. Of course, at the end of the third film, we may find that the villain is perhaps just a tortured hero. Not Kylo Ren though. And not Luke.
The story arc of the first six films is the tragedy of Annakin Skywalker. So, we are back to Annakin... The "villain" will be connected to Annakin. So how can we have a connection to Annakin that is canonical? That is, it follows the story arc of the first six episodes?
i think the answer lies with how smart you think Sidious was. either he learnt everything about the force from his master, and so could create life and prevent the ones he cared about from dying, or he didn't.