If there are any Sombra fans or people who just like the basic concepts of his character in general post a comment below and tell everyone what type of character do you want Sombra to be. Just to be clear: since the makers of MLP are known to take an old villain and their concept and than alter it to something almost unrecognizable to the original, so here Sombra is simply a big dark stallion with a scary vibe to him. From there on, you can let your creativity build his character.

Now, I would like to share my personal thoughts

Since it's probably going to be a mostly-for-kids show, I'd keep his appearances pretty short in exchange for increasing his terror factor to eleven. There's two ways I can see this being done:

1. Sombra is still the ancient king of evil and nightmares and whatnot, but unlike being all hype and no action, you let him show his vileness by manipulating the main characters, putting them through the worst possible situations with very very subtle mature hints that classic golden cartoons always had, nearly breaking their spirits simply by maneuvering them into facing misery. Let him be a major threat to all of 'G5 Equestria'. Of course, he'll get beaten with the classic power of friendship or love. Not only will it keep the series relatively family-friendly, but it'll establish the thought in children's minds that there are people or things that will ruin you, that cannot be reasoned and that you can only overcome with determination and love.

A fitting theme for this Sombra would be:

2. Sombra is like a Fallen Knight or Death from Supernatural. Someone who once fought against disputing rulers and deities. Someone who once burned a whole world tainted with evil, ignorance and delusion, thus allowing (G5 Celestia/Luna/New Name?) to establish a more peaceful kingdom. The reason I compared this hypothetical Sombra to Supernatural's Death is because that's how I imagine him. Not a villain. Not a hero. Just a terrifying force of nature that reaps those he deems unworthy of keeping their lives. Death Itself. Someone that not even Discord, Chrysalis, Tirek, Nightmare Moon and the Sirens wouldn't dare to mess with too much if they get re-used, as well. I can see him having a one-sided friendship with Discord, tho.

A fitting theme for this Sombra would be:

Thanks in advance for reading this wall of text!

I like your second idea because in the head-canon of my story, "Blaze the Pony Tale", Sombra is actually the son of the Pony of Death

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I want him to be one of the Ninjago villains. Introduced in the first episode of a season keeps coming and furthering their endgame, only to be defeated in the end, before slinking off into the shadows to be reformed or as a ally to a new villain.

7331760 Considering G5 is going to be starting anew with a new cast in a new world, I don't think there's much of a chance for King Sombra to appear at all in G5.

Why not a combination of the two?

I prefer the first. In his first appearence Sombra is truly terrifying, as he's able to manipulate ponies through their fears so much that they give up and lose hope. My ideal Sombra would be something like this but with more character, personality and charisma (for example: backstory, dialogues, interactions with the cast, etc...)