>>23561537
Kouki said as much back in 2020, back when Integrate was announced and there was still hope for the franchise.

> “Kaseki no Uta” is a story that takes place far in the future of the Age works, but I think the fact that this was the first work we planned had a big impact on the direction of Age. In other words, although it turned out to be a mirage, we were able to use our maiden work to determine the endpoint of what we would create in the future.

There's a number of interesting elements in Kaseki that seem like first drafts for concepts that would be expanded upon in Alternative, like the 'Dolls', which are clearly 00 Units.