From what I read, the first two things Nakazawa came up with were "snowy mountain" and "transfer phenomenon". Of course, the initial idea for Ever17 was "people trapped in an underwater facility", and the initial idea for Never7 was
so the plot twist wasn't what they always thought up.
Some other sites seem to imply that there was some sort of conflict amongst the staff (Kotaro Uchikoshi using a pseudonym that he's never used elsewhere seems to add credibility to this theory). Time might not have also been the issue, given that Uchikoshi was given a lot of time to write 12Riven (even without the delay due to KID's bankrupcy. Of course, this required him to work in the office for long hours on end, but this is apparently normal for Japanese employees).