Guild Wars 2

Do I have to complete the base games story line to access the expansion zones? Or is there even a heart of thorns story line?

There is a HoT story line and we don’t know if LS2 will be required but it is unlikely to be required.

There is a story to HoT. You only have to own HoT and be level 80 to access it.

When you start the story for HoT it is assumed that you have completed the core story and the living world updates.

Varezenem.2813:

There is a HoT story line and we don’t know if LS2 will be required but it is unlikely to be required.

We know it won’t be required because not everyone with HoT will have season 2.

As Apophis said, the only things required are being level 80 and having HoT. However the story takes place chronologically after season 2 which in turn takes place chronologically after the personal story. The story will assume that your character has experienced the personal story and season 2 (and even season 1).