So far ZOS has done a nice job of this:
- putting some new DLC dungeons and trials in base game zones
- adding repeatable guild quests (Mages Guild, Fighters Guild, Thieves Guild, Dark Brotherhood) to base game zones
- adding holiday events that sometimes involve less visited places in base game zones
- having NPC from base game zones show up in DLC
There's room for more. Which I will elaborate on in future posts.