I don't think it'll be Hydra, at least in the strictest sense. Red herring for something else but likely still sinister.I know this can't happen but it would have been hilarious in a twist if he does kill Coulson and then Nick Fury just shows up and is like 'sup I'm back'.
Slightly disappointed this is HYDRA all over again though. Hope to be surprised.
I hope Rosalind is Valentina so badly