Pretty Cure Mirai Spark/WMG

Hiroko has a greater affinity for antiquity than the rest of the cast.

She doesn't like pink, and yet she's the girliest of the initial Power Trio. In short, she displays True-Blue Femininity more than anything, and it seems clear that she was born in the wrong decade by that clue alone. Either that, or this series is set in the 1950's or the 1960's and thus overflowing with Anachronism Stew.

Someday, the current Pretty Cure will become Barrier Maidens too.

Being a Barrier Maiden is a trait passed down from one team to another. After the current threat is vanquished, the Cures will become Barrier Maidens once the new generation awakens. (Shun will become one as well, being a special heroine.)

Some citizens of the Devil Kingdom are transformed humans.

Word of God: While none of the main antagonists we've seen are transformed humans, some humans have ended up finding their way to that world and (usually voluntarily) being transformed into demons. (The reverse has happened as well.) Additionally, while I'm not naming names, one of the antagonists does have human blood in them.