I don't think there's anything concrete about this per say. We know Nicole wanted to (and well, did) go solo right after PCD (the album), so I guess people assumed she would have officially left the group if her solo career had caught on. 'Doll Domination' is basically the solo album that was meant to be released by Nicole, but reworked to fit the Pussycat Dolls a little bit better. Add to this the fact that Robin Antin was always pretty open about the 'Pussycat Dolls' being a brand, more so than a group per say, I think she might have envisioned an affair where the line-up is constantly rotating/shifting/changing, but the name remains the same - because "everyone can be a Pussycat Dolls/we all have one inside of us". Afterall, that how the burlesque troupe was originally (it had lots of different front runner/lead performer). And obviously, as demonstrated by the few posts earlier, there was a time when she did try to replace the four dolls with new girls and sell it as The Pussycat Dolls nonetheless.