5 Good Fanfiction Ideas, which one should i write?

I always think of good fanfiction ideas randomly, but whenever i try i cannot fully capture what i'm imaging, and usually give up writing a few thousand words in. I am going to post all of the ideas that I had thought of. Please tell me which one i should try write

1 - Dimensional Diamond - I got this idea after reading dash of humanity. Tabitha St. Germane is no longer able to perform the task of voicing the character, so because of the shows popularity they take in a spot on impersonator to now voice as her is inside rainbows head, what if they switched bodies, in each other universe and they can read each other's thoughts cross universe? However, it would be a several minute delay in communicating so it could be very intense waiting for important information. Also, the new voice actor would have to make sure shes using the rarity voice, and rarity saying that she keeps using that voice to not get out of practice for voice acting. Dun dun dunnn.
Now only if i could pull this story off ;c

2. (Unamed) - An uncontrollable force brekkens on equestria, and slowly begins to unleash creatures from tartarus to plague the land and kill all the living species. In desperate measures, the princesses create a universe identical to equestria before it was constructed on. However, due to the size of the area created and the portal specifics (i would think of a better reason) it can only sustain 5 different species to traverse. So in the upcoming three days, there would be large debates over who should be able to go through - Ponies, Breezies, Minotaurs, Discord (poor lonely Draconequus) changelings, Dragons, griffons, and diamond dogs. Sad moments would happen, struggles of deciding, yadayada. I WISH I COULD WRITE THIS!

3. (Unamed) - Remember at the end of season 4 where discord hands celestia that flower? What if that was actually a type of flower which increased the wearers powers by twice as much, keeping it that strength and then making them week? Celestia happens to wear it while raising the sun, making the intensity twice as strong. This unleashes a large heat surge, harming equestria. The chapters would go through the affects of it, as well as ways ponies are trying to fix this.
This was my first mlp idea, never made it ;c

4. (Unamed, again..) - Many different alicorns exist controlling the world, except the only known ones are celestia and luna, for a reason. One pony living in horse shoe bay discovers on a deserted island hides the aquatic alicorn. After him being alone for over 1000 years, he accepts her company. After months of gaining trust, they begin to love eachother. Unfortunately, he dies.. due to a curse removing his immortality temporarily and is killed. Before his final breath, he sends his alicorn powers to the female pony, her becoming the new alicorn. Due to the distraught, she hides out in the same den for hundreds of years. Because of this, she never learnt how to control the oceans and the oceans became waveless (i know moon controls the ocean, shush). After years, she decides to leave in secrecy searching for the alicorn sisters. Everything she had known before had changed, she eventually finds the sisters. They learn of the water kings death, and soon teach her to control her powers. She also learns about the dark side of being an alicorn..
THIS IS THE ONE I WANTED TO WRITE THE MOST! I writ around 20 thousand words for it originally, but due to me personally thinking that it was not good, i deleted it, which i now regret. I want to re write it, im just not sure yet.

5. Mental Institute (i thought of a decent name, yay!) - Yes, this is a human in equestria story. However, the (female) human has a very serious case of multiple personality disorder as well as torrets. After being transported to equestria, and multiple ponies being scared, she is taken to the rulers of the land. After learning the mental problem that is unknown to ponies, she is taken to a mental asylum. There ponies are being tested on.. a bit to scientifically. She learns this, and being a foreign species to ponies they try discovering a bit more. Her having multiple personality disorder, she would have a very different approach to each problem that she would face, making it very interesting in my opinion.
This idea i thought of today, and is a little less developed. Still would be interesting in my opinion.

So which one should i create? I would like to make all of them if i could, but from my past several fanfics i disappointingly got a low upvote/downvote rate. This could possibly be because each of them i had made in just a few hours, and if i would make one of these i would put in around five times the effort. Thank you for your consideration.