VPC management at the organization layer (cross-project VPC) in addition of projects layer
In order to match with GCP Host project/service projects mechanisms (network resource is managed by the host project and used by its service projects), which is usefull for companies having a large number of GCP projects to manage, in different business units, it could be very usefull to be able to not manage VPC inside projects only, but also at Organization layer: an Atlas admin could create an "organization level VPC", and peer it with a GCP host project, and several Atlas projects could 'consume' this VPC, by running clusters in it.
That would permit to avoid having to create plenty of VPC (one per Atlas projects) and establish many peerings, what is technically limited anyway.
Being transparent with you, it would be a massive architectural change to contemplate something like this and as a result not something that is going to happen any time soon.
However, slightly orthogonal but hopefully directionally helpful: we are looking at making it possible to use a smaller CIDR for the Atlas-side VPC at the Project-level on GCP in the future. The reason we use a wide CIDR on GCP is that GCP VPCs are global (an awesome feature) and we wanted to leave headroom for you to grow into any region over time. However in practice we realize there's a middle ground compromise here to be found.