My Hopes for the Pony of Shadows:
Gender - Male
We already have a large cast of female pony villains, so why not have a male pony villain. Sombra doesn't really count in my opinion because he didn't really interact with the main six, either verbally or physically. And the Flim Flam Brothers are antagonists, not villains.
Pony Species - Non-Unicorn
Don't get me wrong, unicorns are great to an extent. But the writers have overused unicorns as villains. I would die to have a villain not be a unicorn. Plus if the Pony of Shadows is either an earth pony or pegasus, it would be more impressive for them to have entrapped Starswirl and the other
Powers - ???
I'm honestly not really sure about this topic. I guess it depends on whether or not the Pony of Shadows is its own species, if the pony is cursed, or if they obtained some power on their own to turn into shadows. I really don't know. Any of those options I listed would work for me.
Location of Confrontation - Castle of the Two Sisters
Who are the Pillars of Old Equestria supposed to be? I'm thinking that they are the legend ponies introduced earlier in the season. It would just make sense to give the audience a little background on the characters before the finale.
Celestia and Luna better get some good reunion scenes with Starswirl, when he is released from captivity. I mean, they have a lot more history together than Twilight does.
When it comes to the Pony of Shadows I am really hoping it DOES NOT turn out to be King Sombra coming back. Sombra is Shadow in Spanish. (Is there a connection there? Maybe...) It would just be a huge flop for me. King Sombra has already had his chance, like twice, and he doesn't need another. Sombra is DEAD to me!