From the name Echoes of Vana'diel, It could be likely to involve Atomos and maybe Kupofried.
If that is the case, the "main cast" would probably be caith sith and Lilisette.

In the XI main characters, we kinda have 3 "travelers" : Shantoto, Lilisette and Iroha ; those are likely candidates. Another case could be made for Lion and Prishe.
Thus Aphmau and Arciela are less likely to be involved.

For the gear rewards, Abjurations are likely, so are Salvage.
XI best gears was JSE, but it's unlikely since the jobs don't match exactly between the games.
Another I would like are RSE but like JSE they don't exactly match the current XIV races.

For mount/minions, we still don't have hume/galka minions ; we also don't any of the sea critters, so there's that option.

For songs, probably Vana'diel march and Memoro de la stono.
It's unlikely but I'd like Heavens tower and Pso'xja. (Movalpolos too but we'll probably get this one with the tribal quests)