I think Snoke is Plagueis for several reasons. Given that both he and Sidious were both sith, it is safe to assume that they both to some degree possessed little trust for each other. If thats the case, and given Plagueis likely also had the ability to foresee the future as Sidious could, then he likely knew or at least assumed that Sidious would try to kill him, and thus in some way faked his death, or even really was struck in his sleep, but used the force to bring himself back to life (which would also explain his face wounds). Following Sidious's betrayal, he went into hiding for whatever reason, possibly still very weak and recovering from the encounter.
Although I am not sure what he may have been doing during the prequels and the original trilogy, I think he finally recovered sometime during Palpatine's time as Chancellor, but was aware that his former apprentice had now surpassed him and he could not defeat him, or perhaps could not find a way to confront him at all, given that he was likely under heavy protection. It is also possible that he foresaw the events of these movies, and, knowing that Sidious would meet his end by his own apprentice, decided to continue to hide until the time was right to return and forge a new empire. All of this would explain his age, scars, obviously non-human appearance, and even why he mainly appears in large threatening holograms to his apprentice, in order to subconsciously instill fear of his power in Kylo Ren so he is less likely to betray him.
Whatever you think about this, I personally like this theory because it also can show how much of a threat Snoke can be. Not only is he the one who taught the Emperor everything he knew, he also managed to evade the Emperors "all knowing" detection for decades, likely taking advantage of his former apprentices arrogance. It is also possible that Snoke did not truly teach Sidious everything he knew (such as how to bring someone back from death), and still has several new tricks that he can use or teach to Kylo Ren (like that awesome blaster stopping move or the mind reading). Whatever the case, I think that there is much more potential if Snoke is Darth Plaguies rather than the son of Sidious, and personally hope that this is the case