I've come up with a number of ideas for new ponies, some of which actually seem like they could be doable, and others which they would never actually feature on the show, only in fanfics.
#1: Blitz: A Unicorn with a very unique ability: he can transform himself into a bolt of lightning and fly at incredible speeds. He can even vary how much of his body is transformed at one time; the more of his body is transformed, the faster he flies. He can simply transform his hooves and move through the air at a leisurely pace or just levitate; he can transform from the waist down and fly at a very high speed; he can transform his entire body and go into what the Spaceballs might call Ludicrous Speed; and everything in between. He would make a great rival for Dash; imagine how Dash would react to losing a flying race against someone who isn't even a Pegasus.
#2: Mimic: A pure white Earth Pony who has no cutie mark of his own. His special talent is that he can peer deep into the eyes of other ponies and basically read their souls, the very blueprints of their lives, and analyze the very essence of their cutie marks. Once he does this, an exact copy of their cutie mark appears on his flank and stays there for about 10 minutes, giving him full use of the full potential of their special talent. He doesn't steal the power; he just copies it and amplifies it to its full potential. He can even do this with ponies who haven't found their cutie marks yet, giving them a preview as well as an idea of just what they'll need to do to find their cutie marks. Also, if their talent calls for wings or a horn, he will grow wings or a horn until the power goes away.
#3: Aurora: Another rival for Dash; who in most aspects is practically her doppelganger, but with a few distinct differences. Just like Dash, she is a very fast and competitive Pegasus, who is also about the same size as Dash; however, she is much more graceful like Rarity and leaves an aurora in her wake as she flies, as opposed to the rainbow contrail left when Rainbow flies. Sounds like a great rival to me, because the two of them are both extremely fast and competitive, and they represent two different atmospheric phenomenon caused by light refracting off water in the air.
Now here are a few that I came up with for a special fanfic I'm brainstorming:
#4: Xenroc: A pitch-black Alicorn with a black widow for a cutie mark. He is the embodiment of shadows, and has power that not even the Elements of Harmony could defeat without help. He can create duplicates of his opponents made out of all the negative feelings in their hearts. The duplicates are completely under his control, and will fight relentlessly with all the power the opponents themselves have (no more, no less). He also has a special power of shadow-puppetry: he can lift shadows from the ground and walls and solidify them; he can create shadowy projections of himself to deliver messages (or warnings) for him, and much more.
Tank: The first one of Xenroc's three henchmen. Tank is a massive Earth Pony with the power to generate a nearly-indestructible suit of armor when in battle. He is so massive that he can cause earthquakes by rearing up and stomping on the ground. He has a classical "moving fortress" fighting style, using brute force and nearly perfect defenses to repel opponents.
Twister: Xenroc's second henchman. A Pegasus about as large as Celestia, he has the power to generate tornadoes and gale-force winds with beats of his wings. He can also fly so fast that it's nearly impossible for his foes to get a lock on him.
The next one I haven't yet thought of a name for: Xenroc's final henchman. A Unicorn with a Thermometer as a cutie mark; his power lets him control both fire and ice, as well as the power to drastically raise or lower the temperature in certain areas.
Psychanazizchou: An Alicorn who is the embodiment of light itself, who appears at the last minute to help defeat Xenroc. His cutie mark is a heart of gold. He starts out as a normal Earth Pony whose cutie mark is a broken heart; because all of the friends he has ever had let him down and abandoned him in the past, he refuses to call anyone a friend, but he still helps out the mane six because he knows it's the right thing to do. In the end, when he sees how much pain Xenroc is causing to the mane six, his empathy unleashes his true power and he transforms into his true Alicorn form.
