Well to be fair they did do a poll about the iterest in a second expansion - that might indicate that they weren't a 100% sure that they would do it.
I even think that one of the developers commented that they didn't know if they would make another expansion or just straight up start working on d4 instead.
I read some cool ideas and suggestions and I noticed a lot of you start with "if there is a second expansion". There will be second expansion for sure and as far as I remember they said there will be even more than one. Because if the support stop, the game is over. With its ups and downs the interest in the game cant last a year without new content.