Outline

The following is a bit spoilery, as it contains the rough outline I have in mind for "Crystal Heart".
Please do not read if you want to be spoilered, or unless I haven't posted anything within a year.

Crystal Heart looks a bit more into the past of Equestria. It is inspired by many works, but ignores some of the comic-lore.
It is planned to follow two story-lines, one in the distant past, following the origins of sombra, and his fall ... the first part is already published, with his creation of the darkness that was left behind in the ponies' hearts at the time the Tree of Harmony came to be.
That negativity is what draws the windigoes, which follow him - which is why snow always follows him.
Still, he is open minded and hopeful, but finds the three tribes already falling out with each other.
He encounters Starswirl, who senses some of his nature, but tries to welcome him, including him in raising the sun. Something goes wrong, and Sombra senses some of his own nature, while everyone else want him gone. He sees everything as lies, and leaves, going further north.
On the way he meets more ponies, encountering greed, and cruelty, as they try to stay alive in a country that is ravaged by snow storms. He presses on further north.
When he arrives in the Crystal Empire, the snow that followed him, is left behind at the city borders, and he finds the first pony that accepts him. He falls in love with Crystal Heart, but when he works up to admit his love, she rejects him; She had given her heart to the city, and he feels sad, and betrayed. He then sets out to steal her heart ... taking her and the city for his ambitions, planning his war ...

The second story line is in the present, with Luna finding her memories about the Tree of Harmony, and a darkness that has been left behind. While she confides in her sister, they start to investigate that darkness, with Twilight helping them. Big spoiler: Celestia gets infected with that darkness herself, and while they slowly succeed in removing that darkness from Luna's memories (and witnessing the ones from Sombra, above), the one in Celestia's grows, blocking the happy ones, and reinforcing the image that Luna is an ursuper, she slowly falls to the darkness.

Luna has to flee the palace, and meets with the Elements in secret. She tells Twilight the conclusion of Sombra's story, with their attack on the Crystal Empire, culminating in their fight with Sombra ... and Sombra infecting Luna with that darkness. Although she thinks that she was in control, it managed to ursurp her memories.

Twilight suggests going directly into Celestia's memories and battling that darkness there ... Luna stands once more in front of Sombra ... alone, if it were not for the rest of the mane 6 helping her. Together they overcome his darkness.

Well, that is the rough outline ...