I mean, either keep it slice of life or, if you're going to go all out, take a page from Shounen anime (DBZ, Demon Slayer, Attack on Titan for you normies), and go right the fuck all out.
Like this.

"As Chrysalis shot out a beam of green energy rocketing towards Celestia a golden shield sprung up and the attack ricocheted. Chrysalis cried out as it struck her and she stumbled back. "Child." Celestia's voice did not waver. She strode forward, her eyes aglow. "Child. I was there when your foremothers knew Flutter Valley and I stood by Queen Rosedust against the darkness eternal and you would challenge me? I give you one chance. Apologize, call off your army and I will allow you to live."

Chrysalis laughed, a cold and bitter sound. "Out of my way, old mare!"

The second blast sent Celestia against the wall. Twilight galloped, scooped her up. Celestia weakly groaned. "My faithful student...I am so sorry...I did not...believe you...that witch enchanted my mind..."

Twilight wiped away her tears. "Don't talk. Rest."

That's when the sun went down, plunging the world into darkness. And through that darkness strode Nightmare Moon. "You dare. You dare, you upstart queen? To strike MY sister, to invade MY kingdom?"

Chrysalis answered by charging her. Nightmare Moon batted aside her horn. "Element Bearers!" The night princess shouted. "What are you waiting for!"

It really should’ve stayed slice of life all around. The characters were at their best when they were in situations where they were relatable. It was the entire reason the first generation of Bronies got as hooked as they did, really. Even in the IDW comic series, where the more epic storylines are more derided than loved, the slice of life storylines have received the warmest reception from readers.

7795392
I really do agree. As a writer myself, I know that thematic consistency is best.

Hasbro wouldn't have allowed it. I believe Lauren Faust originally intended the Nightmare Moon story to be ongoing throughout season one.

7795392
As fantasy, it is banal. As slice-of-life, it has singular charm.

That's what I'm saying! I had a similar thought. The show should have stuck with either epic fantasy or slice of life. Personally, I'd say stick with a slice of life since that was the show's whole identity and where its charm came from. It's like the show couldn't decide what to be.

7795384
Is this from a fanfic, or did you make this off the top of your head? If it's a fanfic, send me a link because that sounds cool!

7795478
Off the top of my head.

It really needed to stay on the slice of life side of things. The characters were what made MLP work, and trying to force them into big adventures broke that badly. The most obvious example is Fluttershy because while she was an excellent character who enabled a number of fantastic slice of life episodes, she really had no place in combat and mostly acted as an anchor on the group in adventures, or was left out entirely. The epic fantasy elements were also a major contributor to Twilight's transformation into a show-devouring mary sue, especially after her refit made her unambiguously more powerful than the rest of the cast (before that Rainbow's flight skills at least forced McCarthy to find ways to mitigate her).

Incidentally, what happened to MLP did make me realize a hilariously overpowered combat character can actually be a major boon to a slice of life show by neutralizing most mortal danger and making combat-focused episodes impossible. In MLP's case, this could have been achieved by dropping Princess Luna into the core cast at the end of the pilot, letting the show explore her character, guilt, and recovery in depth while accepting that she makes the various attacks impossible because she could punt things like Hydras or Diamond Dogs to the moon with a thought.

7795528
True. MLP was never built up as an epic fantasy series considering most of the characters have no place in fights. The only way for an epic fantasy MLP series to work in my mind would be to remake the series from the ground up. Not saying that's the best idea, but just a thought.

7795533
That or a spinoff. Rainbow Dash and Princess Luna both could have been used as the basis for a more action-oriented spinoff, but even then I don't think it would have been a good idea. Faust's worldbuilding wasn't really set up to enable sustained adventure and combat, and it would have been a nightmare to kludge in a more robust magic system to make conflict consistent and engaging.

7795528
Hence why Nightmare Moon (which I headcanon as Luna's "I'm so angry I can't contain my magic" form) vs. Chrysalis would be so epic. Oh well...

7795589
Yeah, that's why I think remaking the series would be the best option. The epic adventures in the main show may work for a spectacle, but they hurt the show in the long run.

7795677
At least we still have fanfics, right? At least that's something.

I haven't actually seen the episode in question, just picking up the key points from the fandom, but after seeing that rewrite, I'll take a shot at one as well. This is intended less as a story and more as directions for an episode.

Twilight spots a shimmering cloud snaking its way among the legs of the ponies present until it comes to a stop a few metres behind the insect queen. A second later, it resolves into Princess Luna, looking very angry. Without a word, she releases a massive beam of magic at the queen, engulfing her from behind, resulting in a piercing scream for a second and reducing her to ashes and melting part of a wall a few metres away which the queen had been facing.

"So, who's next?" demands Luna.

The camera pans across the crowd and suddenly zooms in on a random changeling. Luna's voice over the top saying "How about, thou?"

The changeling is engulfed in blue magic which starts squeezing it harder and harder, the creature looking pained, then suddenly cuts away to the ponies and we hear a wet crunch sound and the ponies give grossed-out expressions.

Repeat on another changeling, this time the others looking a bit agitated. Luna's voice saying "Perhaps I need another example? Maybe thou!"

And a third time, with them looking rather nervous. "Not getting the hint, I see. Thou!"

After the third one is crushed, the rest's morale finally crumbles and they flee.

"Yes! That's it! RUN! Run to the ends of the Earth! Keep on running! And if you ever return, I will kill ALL OF YOU!"

The crowd of ponies look at Luna in a mix of awe and fear.

"Nopony hurts my subjects. Or my sister."

The ponies breathe a sigh of relief.

7796292
Absolutely not. That would have been a terrible fit for the show and made the already poor episode even worse.

7796295
Eh, it was a spur-of-the-moment thing. It had a 90% chance of being rubbish. Looks like it met the odds. [shrugs]

7796529
No worries. It's unfair to hold it against you when you didn't see the episode in question.

That said, I do know how I would have reworked it. Since Luna and Rainbow Dash had basically nothing to do for wedding prep, I would have had them go buddy cops trying to root out what would have ultimately proved to be a very minor threat (e.g. a single changeling sneaking in with the primary goal of being left alone). That lets the episode spend most of its time building a relationship between Rainbow and Luna over the course of their investigative shenanigans and leaves the episode with a good comedic punchline when it turns out they caused far more chaos than the mysterious threat ever could have. That would have fit much better with the rest of the show and set things up to bring Luna into the spotlight more in future seasons since her newfound friendship with Rainbow Dash gives her reason to visit Ponyville on a regular basis.