I read somewhere recently where Yoshi replied to a question where someone had asked about this very thing. Yoshi said that they would love to do larger dungeons with more to it, less linear, more bosses, etc, but it isn't possible with the rate they release content. There just isn't enough time between patches to do so, and he said he thinks players would prefer content released at a frequent rate rather than a longer wait between patches, so they would stick to how dungeons and raids are currently done for the time being.

Yoshi and them also once said there are many things they would like to do, but cannot do until they drop PS3 support. The PS3 already has trouble handling the game. They were going to drop PS3 support by the first expansion, but apparently they decided to keep supporting it through Heavensward.