Allow cluster to define priority of availability zones
Similar to how it's possible to select the preferred region in a multi-region cluster, it would be beneficial being able to select the preferred availability zone in a single-region cluster. This would allow an application to live as close to the cluster's PRIMARY node as is possible.
Looking for the same feature. This will be also quite beneficial in order to significantly reduce the cross-zone traffic costs, since inner-zone traffic is typically free, like in GCP.
Having the ability to define the allowed zones per region would be a plus, with a minimum of 2 or 3, depending on the specific region. And then define a preferred zone for the primary node whenever possible.
It would be helpful to understand what it is about your application that would benefit from. this. Noting that cross-AZ network hops are generally sub-2ms, single-AZ network affinity is usually reserved for the kinds of applications with more niche latency requirements (where latency trumps availability) like ad serving and high frequency trading.
It would help to understand how you would take advantage of this, what it would mean for your applications, etc. Would you have other application tier instances running in the other AZ in the event of failure of the "preferred" AZ? Maintenance is another consideration--one key benefit of not thinking at the AZ level is you avoid pets and force yourself to be resilient for failures and maintenance.