Should I buy now of wait for the new expansion?

Hey guys, I see the game and it's DLC's are on sale and was thinking of picking it up since it looks pretty sweet. I was wondering if I should get it now since i'm not sure how the DLC's work.

I definitely wouldn't be able to get through all the content available now by the time the new expansion comes out, but how is DLC handled? Is there like a portal or something that leads to the new areas so I can go there with my main character when i'm ready and I buy the expansion down the line? Or would I need to start a new game once it's out.

Thanks for your help