In the official G5 synopsis it says Sunny and Izzy will "make new friends out of old enemies."

Who are the only old enemies left? Who are the only G4 characters, possible exception of Discord (unless he gave up his immortality for Fluttershy), who will still be "alive"?

Are they releasing Chrysalis, Tirek, and Cozy Glow to reform them? If so, how will they be released and how will they be reformed?

My theory on the redemption bit is, Luna set them to share a dream as statues, and, after years and years of comically fighting each other, they eventually became friends with each other and truly felt the magic of friendship as Discord did. But by the time this happened, Twilight had already died and the current ruler of Equestria doesn't enter dreams.

As for how they're released? I have two theories on that.

1.) With the Windigos having been destroyed in the S9 finale, Sunny, Izzy, and Pipp feel the need to create a threat to unite the ponies and make them see the bigger picture, as the Windigos had previously done just before the founding of Equestria. So Izzy releases them, but by now they're reformed and become a part of this mission.

2.) Izzy senses friendship radiating from the statue somehow, due to them having become friends in their stone sleep dream, and releases them.

That's just my thoughts. Again, we don't even know the "old enemies" in question are them. It might even just be Discord.

If they do, I hope the redemption is played well enough that people don't complain about it. (They seemed irredeemable last season aside from one brief moment they resisted) But it wouldn't feel cheap at least, as we'd get to see more of their reformed selves, especially Chrysalis.

Though, we don't even know they're using the same voice actors. Already confirmed Tara Strong won't be returning as Twilight, and we don't even know they're using Ocean studio either. They might be using Viz. Michael Yurchak was confirmed to be doing a voice for something straight to Netflix. Some people think it's Sailor Moon Eternal as he voiced Hawk's Eye in the original, but it's WAY too soon for this to be dubbed seeing they usually take a year and a half (though I'd love if it was dubbed a lot sooner). So what if it's the G5 movie?

If they're not using Ocean, and no chance of Kathleen Barr, Mark Acheson, or Sunni Westbrook reprising their roles, and not wanting to recast them (like they didn't want to recast characters from the movie, though they DID recast Sombra), then we might not see them again except as a statue in an easter egg. But if they are using Ocean again, there is a chance?

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I assume this too. If “old enemies” refers to G4 enemies and not new G5 enemies we never heard of, the Legion of Doom are the most logical enemies to return. Either their freed or have been exiled, and decide to help out of revenge against the true enemies.

Of course fans will also speculate they're faking it - even if they're not - since all three of them are extremely manipulative.

If this is true, then there is no hope for this movie and sequel series.

They seemed irredeemable last season aside from one brief moment they resisted

Change is not easy. And villains didn't have a pleasant experience with friendship. It is quite understandable that they would totaly freak out on the first try.

to see more of their reformed selves, especially Chrysalis.

About Chrysalis I would see it less about 'really reformed' but more about 'ponies are keeping her on a short leash (behave yourself or you go back on stone plinth)'