No heroes will die, but there will be close calls that feel like they probably were deaths at some point in the writing process. I'm gonna say this time it's Alphinaud and/or Alisaie, because we might be going for a parallel to FFIV, whose twins also had what by all rights felt like a death scene until they anticlimactically got rescued off-screen.
As for the villains... Zenos is going to survive clashes a few times but bite the dust eventually. We also need to remember a theme that happened in two out of three previous expansions: A currently unknown secondary villain will appear dead but come back to die for real in a trial some time later. Right now we don't have a Nidhogg/Yotsuyu/Vauthry, but reasonably speaking we will have one at some point, and as such we need to remember that Nidhogg and Yotsuyu both had the exact same 'back from the dead' pacing. I'm guessing it's probably the Prime Minister of Sharlayan or something.