I have this idea for a story that follows up my first one, The Valkyrie. It's set centuries in the future and I plan on having it revolve around the Mane Six restoring order to the griffons, who are facing the threat of catastrophic civil war that would not only effect them, but also ponydom.
I do have a problem, though: A character is an alicorn OC.
Now, don't get me wrong. I'm not the kind of person who self-inserts themselves into a story just to live out a fantasy. I think that's rather cliche. This alicorn I've been building up is vain, self-centered, and is not instantly forgiven for their heinous actions (actually, I don't plan on her ever being fully forgiven). However, she holds a seat of immense power and leads an entire nation.
I'm trying to figure out the proper way to balance this. On one hand I have an extremely flawed character who I distance myself from when writing, but I also have to make sure these flaws don't lead her being written as obnoxiously grandiose when addressing her position of power.
I'll give a bit of a backstory if needed for a proper judgment, but what I'm trying to ask is:
What you would do to make an alicorn character good, thought-provoking, and non-Sue?

1269372 Big question. Is this alicorn the protagonist?

1269999
Oh no, of course not! She's a minor protagonist/antagonist. The main characters are the Mane Six.
I say protagonist/antagonist because her relationship with the other characters is a bit strained, and a bit complicated.

1270035 If it's an antagonist, that gives the character more leeway than if it were a protagonist. Mainly because making a protagonist super powerful makes the story boring and smacks most people as more of a personal-fantasy character aka a Mary Sue. An antagonist that the protagonists have to overcome works better because then their powers and abilities (which still have to balanced and make sense in the context of the story) just makes them more of a challenge for the heroes to overcome.

Now it's possible to make a friendly alicorn that fairly powerful that isn't an enemy if you stuff them in the background as opposed to having them front and center.

1270078
I'm considering having her be killed off fairly early on to prove the strength of the true antagonist.
The reason I have her listed as an antagonist is because in the story she led her nation into war against Equestria several times (some conflicts lasting decades), and the result was much unnecessary bloodshed on both halves.
But in the story, those years have long since passed, and she's now requesting the Mane Six and co. to help prevent her kingdom from falling into civil war. The fear she has is losing her crown, and she is not concerned about the effects it would have on her country's inhabitants, and her homeland of Equestria. So, she's not really an antagonist, more like extremely selfish and vain.
... But she does have a backstory that could easily be translated as Sue-ish. I'm trying to make her seem like the antithesis of Princess Cadance, but that comes with a lot of backstory baggage.

1270118 What's her backstory that seems sue-ish? Also I'd hold off an killing anyone yet as the reader needs to have a feel that this is a formidable character, which takes time to establish well, to even come close to having the effect you're looking for. Killing her off early will basically make the reader wonder what the point in having her there was and thus it will basically become an abandoned plot thread.

1270148
I wouldn't kill her off early. If I did, it would be a quarter or midway through.
Like Cadance, she was a former student of Celestia who became an alicorn after mastering battle techniques. She was always looking for conflict and a fight, so Celestia thought her skills might be useful in uniting the battling griffon tribes up north. With help from an allied tribe, she did.
But rather than naming a griffon king, she declared herself the ruler of the griffons and cut all ties with her mentor. Cue long war, ages of conflict, and an eventually secession.
It's mainly the student angle I'm afraid of. I'm going to try and make it obvious she was never her "adored and most faithful student" and that Celestia never forgave her for her betrayal, but it will still cause some problems for people.