BE PREPARED FOR A BIG READ (THIS MAY BE LONG FOR SOME. IM SORRY)
This is an idea i have for what the last DLC could be like. It should go out with a bang! A big bag! It doesn't even need to be this idea but it would be cool to have variety. I am going to describe a little about the plot description and what it could be like.

Plot Description: When loading a save and you go outside, a bright light flashes in the sky and crashes into the ground of the gas lands of Skyrim which has opened a large crater to an underground city. In light of the situation, Molag Bal senses this certain power and rips a hole through to Nirn into Whiterun hold surface. Through this hole, a massive tower of black stone rises out of the ground, dwarfing Whiterun itself and reaching the heights of the throat of the world. With the tower, large centaur type creatures, wraiths, giant serpents, enslaved servants of molag bal, dark skeletons and three entrusted warriors of Molag Bal flying in on dragon back, appear with it.

The tower is Molag Bals battle tower. Though Molag Bal is not physically in Nirn, he named a soul who was killed in 3E his champion due to the power he possessed. He was given the choice by Molag Bal to forever serve him in payment for saving him from death or to live a life of slavery in Coldharbour. The champion chose to serve him, even in death. This person is named.... Mannimarco. He is leading the attack and overall mission to retrieve the power which fell out of the sky. Mannimarco has recruited the vampires, merchants and necromancers.

While the events of Molag Bals retrieval take place, the Jarls of Skyrim have pressing matters. Whiterun is under threat of being taken over by Molag Bals army. The city was given a choice to submit to the power of Mannimarco or die. This is where the choices take place (). So talking to the one in charge of the Whiterun army, you are given the choice of submitting to Mannimarco and Molag Bal or the city will have to defend itself against them.

1) If you choose to submit, all the inhabitants of Whiterun are in slave rags and a little pocket of resistance groups are hiding in an abandoned sewer.
Consequences: Vampires are in the city feeding off the citizens but not straight to death. Skeletons are roaming the city and most buildings are destroyed. If you are in slave rags, you will be treated as a slave, being fed off of. So you can't go roaming around in armour or without slave rags expecting to be at peace. I am saying it will be difficult. If you are a vampire and in the armour the vampires are wearing then you will have the benefit of walking around the city freely. If you wear necromancer robes you will be able to travel freely as well. If you take a note describing you are a servant of molag bal you will be able to get around as well You will hear cries and screams. You will see blood all over the city. The sky will be dark purple. Random burnt corpses on the ground. Corpses in cages hanging from walls, roofs etc. Necromancers will be roaming outside the city with vampires as well. This basically becomes the home base of Mannimarco and Molag Bals army. Trying to release everyone will come at the end, if you choose to of course . You will get access to many exclusive armours to Molag Bals army without fight.

2) If you choose to defy them, be prepared for a fight. All inhabitants will be in danger of being killed as they have to fight for there lives. Quest dependent npc's (ones who cant die and main quest dependant) will randomly be teleported out of the city by the court wizard, with the Jarl and his family as well. This creates controversy and hate towards the Jarl by those who had to stay behind. (the killable npc's) PREPARE FOR A BIG LIST OF FAILED QUESTS AS WELL LOL. Solitude sends guards to aid you in the fight. Some npc's will run as well so you should expect to save lives.
The waves will be as listed: Skeletons, Merchants Vampires. 2nd wave: High level vampires, Centaur things, wraiths. 3rd Wave: High level Centaur things, Serpents, one of Molag Bal's warriors on the dragon.

If you die in the first wave, some npc's die and you are only knocked out. If you die in the second and third wave, you are brought back to health by a Telvanni Wizard! No one died but the city is a devastated mess. You find out that the Jarl of Whiterun (whoever it may be) sent a courier out to Morrowind, requesting service from the houses. You also find that the inhabitants of Whiterun are still alive thanks to the Telvanni and Redoran guards unexpected arrival. You see a couple of Telvanni Wizards zapping the last bit of enemies that are still around. If you survive all the waves, then you will witness the Telvanni and Redoran come to your aid and the city is intact. Molag Bals warrior watches as the Telvanni and Redoran come to your aid. He flies back to Mannimarco, informing him of the arrival of the Telvanni wizards.

Mannimarco withholds the rest of his army of raiding the rest of the cities. More Telvanni and Redoran go to each city defending them from Molag Bals army.

So that is a snippit of my idea. This quest ( i would like to think ) starts after 100 in game days when you hit level 20. Level 30 and higher and it starts straight away. I like time dependent quests. They are fun.

DLC:
- Telvanni wizards and Redoran Guards will be roaming Skyrim. Telvanni wizards will have a spiteful attitude. Redoran guards will have a more honorable attitude. With a little spite.
- Once a in game day, skeletons and wraiths will spawn outside each city. If you don't want to fight them, the Telvanni will just [censored] them up.
- The sky will be forever purple and grey when entering Whiterun Hold. The grass will be dead. Trees will be dead looking. It will be windy and when it rains it will be purple ice shards. This rain makes your stamina decrease faster when running. Unless you choose otherwise.
- A lot of cool armours should be introduced.
- Skeleton variants, wraiths, centaur type creatures, giant serpents and 3 warriors of Molag Bals riding dragon back trying to find the power the fell out of the sky.
- Random encounters from the dragon riding warriors who are seeking the power. They will land and ask you if you know of the location of a light that fell out of the sky. They also ask other travelers as well. The travelers will also be cowering while answering the questions.
- Further in my idea, you get the choice of keeping the tower for your home with the whatever remaining creatures that are on the ground that protect you. Choice to turn the purple sky off or on. Allow "Purple Rain" and change it's effects. Effects such as: Faster magicka, health or stamina regeneration. Do the opposite to enemies. Allow all enemies up to level 40 to calm when outside in the Whiterun Hold. Allow "PURPLE LIGHT" and change it's effects. Effects such as: Burning enemies within Whiterun Hold while this option is on. Paralyze enemies for 60 seconds every 5 minutes. Insta-kill enemies below level 20.
- While the complications of Molag Bal are happening, the moot happens. They need to find a high king and fast. This is just a discussion. At the end of the main quest, if you decide to keep the tower, the Jarls join each other at Solitude and name Molag Bals battle tower a landmark and an official city of Skyrim. I suppose I would call it The Black Tower of Skyrim.

I have more ideas of what kind of quest the next dlc could have. What do you guys think of my quest idea? Any further ideas you think i could put in?

...Um...I think the Oblivion Crisis ensured Molag could not do that...

We already dealt with Molag Bal worshippers in Dawnguard.

It was a great Idea, though due to in-game history most of that can't happen.

This contradicts so much lore established by the previous games.

Well this is just my idea of what kind of stuff the new quests could be like. I would love for something like my idea of Molag Bals battle tower becoming a city of Skyrim. I never really studied the lore and i know mannimarco is dead, if i remember correctly after reading the usep. But I think Molag Bal needs some love!

Oh i also edited some of the ideas in my next post. I knew this was a lore killer my idea but I thought something like this could happen.