Purely speculative discussion of support characters. I'll begin with Sunburst.
>What does their cutie mark mean?
Bright ideas. Get it? Sunburst. Dawn. "It dawned on him."
>What would their corrupted version be called? i.e. Celestia and Daybreaker.
Dawn Dealer. He's solutions come with a price.
>Invent an episode synopses
Alternate timeline of The Crystaling sans Starlight Glimmer. Imagine a rumpelstiltskin version where Sunburst offers to fix the crystal heart, in exchange for Flurry Heart's alicorn magic.
Shining Armor
>What does their cutie mark mean?
Protection and Fealty
>What would their corrupted version be called?
Black Armor?
>Invent an episode synopses
Flashback before they were married. Shining Armor doesn't believe his infatuation with Cadence is genuine and tries to break her spell over him.
Unicorns are the more interesting race. I'll try something else.
Spitfire
>What does their cutie mark mean?
She's hot headed.
>What would their corrupted version be called?
Volcan't. (idk, something uncontrollable)
>Invent an episode synopses
Ember wants the dragons to compete against the wonderbolts.
If any pony can coach a team of dragons, it's Spitfire.