Zenos is probably much more than a fallen prince with an artificial echo and his character will severely change the more he pursues his Final Days dream. If we don't kill him for good, he will probably become an antihero. Probably something like Gaius.

Nero will make an appearance in the Weapons trials storyline.

Ryne and Gaia will either both die at the end of the Eden storyline, or they will stay relevant and somehow join us in the source.