I've been thinking about how in Otona Pretty Cure, one of the main and obvious takeaways is that the events of Futari wa Pretty Cure Splash Star, Yes! Pretty Cure 5, and Yes! Pretty Cure 5 GoGo! all occurred within the same universe.
That's not exactly a new revelation, especially since the Pretty Cure All Stars films exist, but in those cases, it seemed to be more of a non-canon affair; that is, not directly tied to the events of the TV anime.
But I also think that, back in the original Yes! Pretty Cure 5 and its sequel, they made slight hints that it took place in the same universe as Splash Star. Whether or not this was intentional for OtonaPre is up in the air, but I still think it's neat.
In the 28th episode of the original Yes! Pretty Cure 5, Komachi and Karen are checking a phonebook to see if there are any locations that can provide ice for their confectionaries. When the anime shows a page of that phonebook, if you read the Japanese text carefully, you'll see that several of the locations are named "Yuunagi" (夕凪).
If you recall from Splash Star, "Yuunagi" is the name of the town that Saki and Mai live in (and also the last name of Tsubasa Yuunagi, Cure Wing).
Karen also mentions that they've been checking locations that are "farther away," and since their goal is to try and get ice, we can assume this means Yuunagi is within a semi-reasonable driving distance from wherever Yes! Pretty Cure 5 takes place.
Now, by itself, this may not mean much, perhaps Yuunagi is the name of another unrelated place. To counter that, I'd like to go back to the phonebook picture. In the address lines for each location listed, the address is written as "Unabara-shi Yuunagi" (海原市夕凪), or rather, "Unabara (Deep Sea) City — Yuunagi."
Unabara City is also a location referenced in Splash Star. Yuunagi is basically like a town within the sector that Unabara encompasses. Unabara is written on signs in the actual anime, referenced to in official material like the website and print stuff like the Memory Book, and was spoken out loud at least once to my knowledge, in the Splash Star film. The Yuunagi mentioned in the phonebook is the same Yuunagi that Saki and Mai lived in.
Another small piece of evidence is that, in the 36th episode of Yes! Pretty Cure 5 GoGo!, when the girls get invited onto a game show, we see students from different schools competing, and in one shot, we see girls wear school uniforms that look very similar to the ones Saki and Mai wore while attending Yuunagi Junior High.
So in short, I believe that it has always been referenced since the original Yes! Pretty Cure 5 that it took place in the same universe as Splash Star, and this is what OtonaPre was built on.
