In the HoT zones, so much of what happens in those areas are group meta events, or group content in general. I suggest when you release the new Expansion, that you change all the content in HoT to not be group content at all, including the meta events. I also suggest that you change the meta events to need massively less people in order to complete.
If you leave it as it when the new expansion is out, you are leaving an area of the game that will not be enjoyable for the players in them when there are so few people playing.
So please take this into consideration.