Oh boy. Old stuff. The original Possession sequel draft was found. · 10:34pm May 6th, 2019
Going through some old junk in my work folder and I actually found the original draft of what would later turn into the sequel to Possession, Last of the Dark Ponies.
Wow, I had clearly different ideas. 
I had vague memories of writing this draft, I think the only things that survives from it is that the story begins with Private and Barricade's birthday party and Sombra arrives at the party singing the birthday song, but I had mostly forgotten about it when I really committed to drafting up the sequel.
It's very clearly written when I was just beginning to write Possession and shows far more that it's based on my original plans with Possession, like Private's possession happening in Ponyville and eventually Sombra is banished out of him via the Crystal Heart.
So here is what almost might have happened. 
-It was supposed to happen in season 5, I wrote Cadance as pregnant with Flurry Heart.
-Barricade clearly did not take part in Possession, he arrives sounding far more like the smug jerk we met in The Time Keeper, but is reformed, thanks to Applejack and the Crusaders, showing that I had been working on the ideas for In Old Manehattan already. He only knows Sombra as the "jerk who possessed Private" Most of the ponies are surprised he's showing up and is good now, except his parents who promise he's on a leash now. Barricade isn't even that remorseful, bit regretful but still clearly needs to be told to behave.
-King Sombra arrives singing the birthday song and everybody is surprised he's in a body, clearly indicating my original plans that he would still be a spirit after Possession, and he doesn't even abduct Private from the party, just leaves after saying he just wanted to wish Private a happy birthday, though he leaves with a ominous "We'll talk later.
-They try to trap him by letting him talk "alone" with Private but sees through it and simply plays up Private's fears of his parents finding out he's bixesual, so he panics when seeing them and then gets captured when not being watched.
-Sombra is actually obsessed with Private without knowing why, just feeling a strong need to be next to Private. This was an idea I toyed with and hinted at as a possibility in the actual Possession story when I had Sombra say he infused a bit of his own essence into Private back when he was a colt. I dropped it in the end, instead just letting it be something Sombra can use to connect to Private's mind easily, without even using his dark magic.
I believe my plans were that Sombra would be dragging Private all over while trying to figure this out, with the Mane six and possibly Barricade (to prove he was good now) hot on his heels.
Frankly, reading this draft I'm glad I went with Last Dark Ponies, this was clearly just Possession v.2 just with Sombra living now instead of a spirit. 
That's probably why I ended up dumping this draft, it was just too similar to Possession in structure and ideas, only keeping the birthday party and the birthday song because that was just too funny and too creepy not to use. 
It does sound like you made the right choice writing Dark Ponies the way it came out. And yes, Sombra singing Happy Birthday = way-the-fuck-CREEPY
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I noticed in the original draft that this was also back when I was still trying to figure out how I wanted to write Sombra, yet already I was doing with him what he does now, enjoying it way too much to freak Private out.