Demashita! Powerpuff Girls Z/WMG

The anime is set 40 years or so after the original cartoon, and the girls are descended from their original incarnations. They all moved to Japan for some reason.
Would at least explain why the PPGZ aren't sisters. The original PPG all married different men. In Japan, for some reason... maybe they developed a strange collective fondness of Japanese guys in their teens or something. The PPGZ didn't directly inherit their ancestors' powers, but the incident with the Chemical Z activated them, explaining where the theme-colours and Superhero names came from all of the sudden... from their genes, of course.
 * Maybe when they grew up, Townsville wasn't quite so eager to have their city trashed by moody teenage girls every time someone broke a nail/got dumped/got an A-? Or perhaps things got Darker and Edgier as the girls grew, as is often the case. But you know what country is used to rampaging monsters/town-leveling villains and loves color-coded superheroes (especially when they're cute girls)? Japan.
 * The Powerpuff Girls didn't just move to Japan they also traveled back in time. As evidenced by the existence of the Oedo Chaki Chaki Girls.

There are Bunny and Bullet in PPGZ, just not as the PPGZ and indirectly.
Since this was not stated by the creators of PPGZ, this should be a WMG. Bunny's PPGZ counterpart is Miko, Himeko's big sister, while Bullet's PPGZ counterpart is Kuriko, Momoko's little sister. They sure have a lot in common, with a few twitches though.

Miko Shirogane is one of Bunny's fantasies.
Bunny wanted to be treated with love, so she became Miko, who's all perfect and well-respected. She was later driven to hatred when she saw the PPGZ because they remind her of the PPG who don't give her as much attention as she had now.

Ace and Ivy will date after the events of the series
Well, that's my guess anyway.