I guess it's a way of just introducing a new antagonist. Aquamarine I suppose could fit the bill, but she feels very… I would't say one-off but I don't think she's ever meant to make many appearances.

Besides her, we really only have the Diamonds. And it seems like they won't be actual villains to face until we get closer to the end of the series. (Assuming it ends once White is dealt with.) So I guess it's just a way of giving us a villain that's a big threat, but not Diamond level big.

That's a legit scouter. And spikey hair. Emerald is actually Raditz. That is just bloody perfect!