Saving Oracle Marieve *is* the Summerset prologue quest. And it does indeed end there, in Skywatch. The prologue quests mostly don't even bother to start you on the first quest of the new zone.
This is because, in every case, the prologue quest was released when the new zone was not released yet, and there was no new zone to go *to* (and indeed, players can do the prologue quests without even having the DLC to which they refer.) So, technically they're one-off, but they frequently span multiple base-game zones - for instance the prologue quest for Elsweyr requires no less than FOUR Delves as well as its own new one-shot area.