Sun/Moon Postgame will have a "Z-Episode"

You travel around with Zinnia to each region (locked to 1 city and a nearby route) in search of Zygarde cells.
Gym leaders from previous games make an appearance, but are not able to be battled.

cities you can go to are via cruise ship, so you can go to Vermillion, Olivine, Slateport, Canalave, Castelia, and Coumarine.

From Coumarine, you can acess the route before it and enter the Kalos power plant

In Vermillion, the space that was being prepared for construction is finished, and there is a clothing store there where you can buy outfits closely resembling characters from other games.

Canalave city library has lore about the "other universes" Zinnia was talking about in Oras

More to come