I'm actually ok with Discord's reformation, as it's really not all that different from what happened to Q, who Discord was more-or-less based off of in the first place. The circumstances were a bit different, but at the end of the day, Q stopped being a full-fledged villain, and instead usually ended up helping the main cast out while trolling them at the same time, which I can totally see Discord doing in season 4 onward.
As for other antagonists, it's more-or-less already been stated above. A lot of people are suckers for a good redemption story, and given the fact that love and friendship are sort of the main aspects of the show, reforming villains makes sense, as opposed to defeating them and forgetting about them.
I'd be okay with Gilda being reformed, seeing as she's barely even an antagonist, she's just a jerk who doesn't know how to be a good friend. I'm sure there's an episode with a good moral that could be made out of that scenario. It'd be interesting to see what happens with Lightning Dust. Do we even know if she was kicked out of the academy, or just demoted? Either way, I can maybe see her resenting Rainbow Dash and trying to sabotage her attempts to join the Wonderbolts, but I could also see them going a different route, with Lightning Dust getting Dash to help her become less reckless, so that she can have another chance. It is a kid's show, after all, and I can see scenarios that could have a friendship moral attached to them being more likely.
As for Chrysalis, I'm not sure. The fandom has a soft spot for her, and I'm sure plenty would be okay with her being reformed as well, but I'd like to learn more about her and the changelings first. As of right now, I'd like to see Chrysalis being a recurring villain instead of being reformed, at least for a while. Between the wedding and the comics, she's proven to be one of the series' darker and more threatening characters, and I'm sure there's still a lot we could get out of her being evil.