I think we all whats one of the big problems with this movie is the high setting. If you change what would it be?
I'm thinking of going to beyond the gates of Tarturus to find the ones who attempted to murder Cadence and Shinning Armour. Celestia disguise the Mane 6 as residents of the other world. While looking they meet a bunch of colorful characters and discover a conspiracy that can destroy their world and Tarturus.

And yes there might be a love interest but that's just for humor. It involves Pinkie.

It's a nice IDEA!

Too bad Hasbro will unlikely do that because MLP is mainly a girl show.....too manly untapped potential....

2294575 Sure is a better idea, if Hasbro dosen't mess this up.

2294575
This.

2294619
Not a fan of Skyrim but sounds like you put a lot of thought into it. Maybe I should do the same with my.

2296706

I was going to make a fanfiction out of it...

... Then I took some laziness to the head.

2297061
Yeah. I have a good few ideas for fanfics. some suit games better like a Commander Keen crossover. But I'm not a good writer.

Other facts about my idea.
Elements of harmony:
They are used but only pretty early on during the first fight with the villain. It doesn't go too well as he gains a soul and becomes insane after the blast. They are also the keys to the disguises the ponies wear in the dark world.

Villains
The main villain is a lawyer called Baphomet, a soulless monster and right hand to the dark one. He's completely emotionless but a quick thinker and killer. His henchmen a four lawyers who are complete monsters, taking enjoyment from every cruel thing they do. I already made a blog on them. Their plan is to increase the tension of the light world far enough for leaders to open a convenience on the matter. Then they'll use a doomsday bomb to destroy the meeting and hopefully start a world war. They all seem to be afraid/hate Alicorns.

Mane 6
The Mane 6 are simply there to find Sunset and stop the demons from any plans they are planning. While Twilight and friends are just looking for clues, accidentally impressing the demons with their skills, and catching the attention of some powerful people. Twilight however is having doubts throughout the adventure about her new title and the idea of immortality is making her feel like depressed about the idea of losing her friends. This isn't help by the main villain also being immortal and taught her about how pathetic are friends are to her. At least she has Spike to keep her mind in check.

Sunset Shimmer
During the time before the movie, Sunset was tricked into coming with Death and trained, beaten and just plain corrupted into a harbinger for the destruction of the light world. After being hit by the elements, she regains her memory. All of it. During the rest of the story she struggles to figure out whats happening in her life and just wants to go home. However the lawyers don't want any witnesses.

Supporting Cast
Just a few demon OC. While not having much screentime due to being a movie they do have personalities to make them unique. Vargary is the environmentalist leader who doesn't like the other demon races. Cycloid is the head of security and prison warden who takes enjoyment in psychical violence and is the most serious of the group. Gideon is the head of the imps and is a very go-happy guy, although we learn he's not really that happy. the Serpent brothers who are just the three stooges. and Makron a realist who's looking out for both sides.

Climax
After the the heroes and anti-heroes sneak on the villains ship they split into two groups. One goes to the bridge to distract the lawyers while the other goes to find the engine room to sabotage it. Baphomet immediately ambushes them and demands a fight with Twilight and Sunset for the simple reason "You remind me of those two freaks of nature." An fight scene follows ending with Twilight stabbing Baphomet in the eye with her horn. the cloaking is deactivated and the Princesses find them and throw the ship, with the lawyers the only ones left onboard into the sun. Twilight is devastated that she had to take someones life but Cycloid(the only one who didn't like the ponies) shouts at for her thinking less of herself for not saving someone, especially one as evil as Baphomet, and says congratulates for everything turning out as good as it did. the ponies and demons say their goodbyes and the two worlds went back to normal. except for Baphomet who actually was killed off in this story.

And in case you ask why Celestia and Luna don't go. the dark one will know that it was them if they set foot and won't hesitate to destroy their world. and for songs: maybe a villain song for the lawyers, an opening song, a song when they get to the city and most importantly, a duet with Pinkie and Gideon.

Well that's all I can say about that.