Sidious looked like the Emperor in hologram. Of course he would be the same as Palpatine.
It was so obvious he was manipulating the trade federation and Naboo into a conflict, and he was behind the creation of the clone army and both sides of the war in the Clone Wars.
He fooled everybody including the entire senate and Jedi council. Even Yoda did not notice the duplicity of the supreme chancellor. He tricked Padme into putting him into the office in the first place. He tricked Jar Jar into giving him the powers of a dictator.
The story only works in the prequels because good is dumb.
As for predictions go. I kind of figured Rey would be a Skywalker, just not adopted. I also figured Finn was force sensitive since early in force awakens. I never guessed Rey was a Palpatine since they just made it up near the end of filming the movie, how could i have seen it the writers did not plan it.