Who is Rogue One's mystery Sith?

Given the Red Guards on either side of the room this Sith is present in, I almost want to say it's Sidious. Perhaps Sidious could be kneeling to Plagues. That I feel, is more likely than seeing Darth Maul's return. 

It's most likely Vader kneeling to Sidious, but I couldn't say.

I don't think it's Vader, like Gav said, he never wore a cloak, always the helmet. 

I concur.

This movie is set after Revenge of the Sith... and a good many years after and is thus set i would assume months before EP 4 A New Hope.

At the moment we cant be sure what this could be, or even if its set on the Death Star at all.

Thus Vader has his full suit/mask and so i would not see the need for him to have a cloak. 

I cant see Sidious kneeling to anyone apart from someone who is higher rank to him, which can only be Plagueis but as Revenge of the Sith stated... Plagueis had died... the expanded universe shows that Sidious killed his former Master... this fits with Sidious greed and desire to rain supreme..  i dont think he would want any part of a Bacta Tank that contained his former master... he has no Loyalty to anyone but himself.

He would also know of the Treachery that has occurred in the Sith and it would not surprise me if he has a Back Up Plan to Vader.

The Imperial Guards are associated with the Emperor but who is to say they cant be Body Guards to other High Ranking Members of the Sith/Empire.  However in the Canon of he movies, who else could that be... and would they introduce such a Character when this movie is set to Tie into Star Wars EP4

I would thus Assume the Bacta Tank is Sidious and he does not have to be on the Death Star, i would assume the Emperor uses this to extend his Life.  I would assume this Secret he Guards

Maybe the Cloaked Figure would make a Cameo and be someone we cant recognize or maybe we never see the Face....  Maybe this could be Snoke?

But we would not know it to be Snoke... only EP8 would reveal that because i cant see them giving away a major Plot to EP8 the year before the release of that movie... but thats not to say we cant have a cameo from a character who could end up being revealed as Snoke!  Or pose as a Red Herring to lead us to think so.

The other theory about Snoke being in the Bacta Tank, again i cant see them showing that... but it could be shot on another place apart from the Death Star that the Cloaked Figure is on, and who can rule out the Cloaked Figure becomes Injured in future and seeks out the Bacta Tank above to heal himself... aka Snoke! (being the Cloaked Figure)

But i think Snoke would be Sate Pestage or Sim Aloo myself....

But if Snoke is not... thats not to say the cloaked figure cant be Pestage/Aloo.

Perhaps Disney will use the character of Galen Marek after all.
This guy might actually be Darth Vader his "secret apprentice".