Depending on customer networking configuration and demand for VM executors, it can be challenging to allocate a single subnet that is large enough to meet demand and allow room for expansion. It would be helpful if Circle accepted multiple subnets when launching VMs, as it's often easier to add a second non-contiguous subnet than create a new large one and migrate all usage.
While subnets could ideally be in multiple availability zones, I realize this makes cache EBS volume management more challenging. Requiring that all subnets are in the same AZ is a reasonable restriction for an initial version of this feature.