There's too much we don't know at this point.
We also don't know how much of the original story will be relevant for ARR, however they did keep the main story writer from 1.0 in ARR so I'm expecting continuity.

I would be surprised if it's Siren, especially because we know she's boss content.

I think it's highly more likely to be Cait Sith. We know Cait Sith will be in ARR, and Atomos is in XIV similar to XI, and in XI Cait Sith was our guide through Time and Atomos (we still don't know how Atomos is involved in the 7th Umbral Era, it could be the same with Cait Sith)
Cait Sith is also linked with Time, Loisoix teleports us through time, the echo is about going back in time.