Well if they have gone with the failure ending being the canon one, it wouldn't be the first one to do it as that was what was used in the Aftermath/Aftershock/Afterlight x-com alike.
Things are certainly open enough that it would be quite possible to go with the Temple Ship having been renegades of the greater alien empire and that was the great threat that they were wanting to acquire humanity to fight.
For character from the first game making a reappearance, I don't think Shen is likely to be one, due to his age at the time of the original conflict and this being at least 20 years later. Vhalen might be a good one to reappear, though I'm somewhat getting feelings of a Judith Mossman situation cropping up if she does. Bradford might be interesting if he returns, likely somewhat bitter about the world surrendering, though hopefully he's gotten over his fascination with dashing.
From the visuals, it looks like it will be based of the same engine, though I'm hoping that the turn being split into three actions is one of those things that is modable.
Also, how many old-com players on seeing the snakeman in the trailer immediately started watching for the happy-fun hug-buddies to show up?