Considering how long it has been I don't think we will ever see a Cloud Key Gen 3/3+.

Ubiquiti's Cloud Gateway Max or Fiber seems to be the modern replacement since they do the job of the Cloud Key while also serving as your router and firewall.

I'd be surprised if Cloud Key gets another revision, it's a dead product line as I see it. The main purpose was to provide an appliance alternative to running the controller on your own server, which is now unnecessary for most users because the routers and NVRs can all host their own controllers.

The remaining market for such a product is people who are running UniFi switches and/or APs but not the router and yet still want an appliance, which is not a large space. Most of that market either has a random server they can run it on or is willing to throw together a Raspberry Pi controller.