I think the Blue Fairy may have some connection with him. I mean, this is obviously a flashback to the time BEFORE he met Rumplestiltskin. The only character old enough to appear there is Reul Ghorn (of course Zoso might have ressurrected just like Rumple when Belle and Neal brought him back, but that's doubtful).

I'm happy they're gonna bring hi back because Rumple is such a complex character and I feel like there's so much we have yet to learn about him and his past and his struggle with his addiction to magic and about his time before becoming the Dark One. Also though about the time he was the Dark One and making tons of deals and ruining a lot of people's lives

Maybe Rumple coomunciated with his ghost and acted as his apprentince, in 3.18 it's been stated to talk to the dead you need the murder weapon and the murderer and Rumple killed Zoso and had the dagger he used to kill him with, so he could have easily talked to his ghost

What if the DO dagger was just a regular knife that Zoso owned but then some kind of ritual happened at the DO vault and the dagger became a physical manifestation of darkness and Zoso became a slave to it (dagger is the parasite and Zoso is the host)?

What if he wrote the magic book used by Regina and Cora numerous times.

I think it's interesting that A&E are going to take a deeper look at the DO mythos.  And this could, very well, be another setup for a future story arc.

What if the Blue Fairy was a love-interest for Zoso before he became the DO and then BF is secretly bitter at Rumple for killing Zoso; even though the BF knows she can't be with Zoso because he's the DO (forbidden love). So she gives Bae the magic bean so the DO will go to Earth and the presence of her forever lost true love (Zoso) is gone.

Some interesting theory juices flowing here!

I personally figured this flashback would be to in between the flashbacks of what I think was 1x07...the first Rumplestiltskin backstory episode-forget what it's called aha. As in, a flashback to the time that Zoso was with Rumplestiltskin before he became the DO and while Zoso was DO. 

I also like the theory that Rumple talked to his ghost, however. 

And I hope Zoso wasn't the first DO, or at least that they don't set that up yet! It would close off so many possibilities...like, WHAT IF MERLIN WAS A DARK ONE ;O

IDK I'm just grasping at straws there but you get my point I hope

Cool, it would be interesting to see what his return is about. I always though that Zoso was an interesting character.

Or what if the dagger exists because Zoso and Rumple were never supposed to possess the Dark One powers?