So the following question was asked in the latest Ask The Devs segment
Q: What happened with the Abyssal Maw dungeon that was supposed to come with Firelands? – Maryjanee (EU-EN), Espiritu (NA)
A: Our initial plan for this raid tier was to have fewer bosses in Firelands and a small number of bosses in the Abyssal Maw. As we looked more closely at Firelands, though, we realized that it deserved more bosses. We also got excited about designing item art (and set bonuses!) that were very fiery in nature, and the Molten Front questing area was turning out to be really cool, so we ended up piling more resources into Firelands. That led to the decision to focus on one strong theme (fire), rather than a more diluted fire-and-water theme.
The case for Abyssal Maw pitch was that we could reuse a lot of existing assets (the fights were to take place in a giant shelled demigod like Nespirah), and while we are willing to do that, we thought Abyssal Maw would just pale in comparison to the magnificence of the Firelands. So, we put all of our eggs into that one basket. We’ve decided for now that the Vashj’ir quest line along with the Throne of the Tides dungeon does a pretty good job of finishing the Neptulon story.
Now I completely understand that splitting resources between the two raids for one tier would probably take away from each of them. Imagine having Tier sets that were a mix of fire and water, how odd we would be looking as a group. Also I know many people don't like this, but I have always enjoyed the idea of 1 raid per tier it makes you think you are actually accomplishing something. Now I understand the idea of having multiple instances especially maybe in the first tier to give options and an option for story progression. Yes there can be bosses that can be blockers, but then you just have to design the instance to have options so this doesn't happen. This is actually exactly what they have done with the Firelands.
Now speaking to all of that I understand why Firelands will be our option come 4.2, and I am extremely excited to take part in everything it brings to us. There is a but though. How can Blizzard actually say they are happy with the way the Nepulon story is at? I sincerely hope they mean for the moment, because being taken away by Ozumat and leaving that unknown what has happened is not a good ending. So good on Blizzard for focusing themselves, but I just hope they do the right thing and finish the Nepulon story correctly