Possibly unmarked Spoilers Below
Idol will be the one replaced by Chrysalis instead of Cadance during the Wedding Arc.
As of the Ponyville Arc, all of the Mane Six have met Idol. If the wedding marks the point where canon gets thrown out the window, then it would be an opportunity to have all the elements of harmony work together on exposing Chrysalis. And this would work better with someone they've all met before, rather than someone like Cadance, who only Twilight knows.- Jossed. Chrysalis infiltrates Canterlot by replacing Cadance, as in canon.
Topaz will be the one to defeat Chrysalis.
It would be cool if Idol could be the one to beat Chrysalis, but brainwashing will likely make that impossible. Topaz, on the other hand, knows more about changelings than any other pony in the story, at this point. She'd be fighting not just for Idol's freedom, but also for the possibility for love and understanding between Ponies and Changelings, a metaphorical echo of what she's been doing her entire career with educating ponies about bugs and insects. That kind of thing carries a lot of weight, narrative-wise.- Jossed, at least in so far as this happening during the Wedding Arc goes. Topaz and Idol Hooves never attend the wedding because they're too busy having sex, and enjoy themselves so much that they completely miss Canterlot being invaded and the invasion then being repulsed.
Chrysalis is a former minion, experiment or would-be rival of Sombra's.
We know that one of the Orders that all exiled changelings get is to stay away from the Frozen North, the area where Sombra once ruled over the Crystal Empire. We also know from Chapter 61 that Chrysalis wasn't always a changeling, but was instead transformed into one by some Black Magic — and, from that same chapter, we know that the curse can be broken temporarily by the magic of Cadance. Whose magic is uniquely similar to that of the Crystal Heart. With these facts in mind, it seems logical that Chrysalis herself has some kind of connection to Sombra, and either he transformed her into her present form for some reasons (or perhaps it was a side-effect of an failed attempt by her to fight him?), or she did it to herself by meddling with the Crystal Heart in some manner. Regardless, she clearly doesn't want to risk falling into his clutches again. Changelings are a Slave Race.
Everything we learn about the relationship between Chrysalis and the changelings in this 'verse makes the relationship entirely uneven. The changelings have their own independent minds and personalities, and they reproduce on their own accord, thus removing the two primary roles of the Hive Queen. In contrast, Chrysalis reigns over them like a dictator, complete with the ability to level powerful psychic compulsions on them, including setting them up to very literally self-destruct if they try to physically harm her. Additionally, they only can recognize her as the queen when she's in her Queen Changeling form; when her curse is temporarily broken, the psychic connections she has to them are broken in turn. This suggests either of two things:- Changelings are a natural race whom Chrysalis has conquered and corrupted to serve her, similar to how Sombra enslaved the Crystal Ponies.
- Changelings are an artificial race that Chrysalis created through the power she gained after her transformation.
Spike repeatedly setting the coffee table on fire, which Twilight Velvet blames on the family's mahogany-scented furniture polish, has something to do with the dragon sneeze trees from the episode Princess Spike.
Plant product that allegedly causes dragons to set fires, plant that is known to cause dragons to set fires, isn't it obvious?