You said you wouldn't mind ideas, just not character profiles, right?
(I really don't want to annoy you, I just have so many questions about your work and I know you don't want to answer them.)
(Really sorry for the next part.)
I have an idea to include the rainbow of Darkness (The fact it was never brought up in the show kind of irks me); basically the Mane 6 or whoever use their rainbow magic on a villain (Tirek, Sombra, Grogar, whoever is evil and good at magic.) and they use their magic to contaminate it, turning it into a dark variant of the rainbow which can contaminate the magic of every creature, turning them into monstrous beings.