Well this was a very fun read I wonder, with cute little sissy Thunderlane still in his diaper. I wonder if Rumble will also revert to that other sissy form he had in the story. I wonder if all of this is part of a wider plan of Zecora, I wonder who could be next Sunburst, Pipsqueak, Poky Pearce, Caramel, Breaburn, or Doctor hooves could also be potential targets.

I kinda want there to be a sequel where Zecora ends up regressed herself

I'd love a sequel where Zecore re-regresses Rumble but this time he goes full transformed into a true filly not a femboi. I'm sure you can find a new ingredients Zecora can get like "The happiest tear of a colt truly embracing being fully transformed into his true female self" or something like that. Have it where he wakes up a full female and after a dream wear he was "possessed by a female counterpart of himself" and at first he tries to fight it but as the days go by he begins feeling great about being a "she" and starts adapting and enjoying "her" new self and end it the same way Thunderlane's story ended where Rumble chooses to stay a female and Zecora get's her last important ingredients whatever it is and we finally find out what kind of potion she was creating and it's purpose.

I liked these stories and i'd love another sequel.