It would be nice if there is a possibility to have listeners listen on more than one port.
In my scenario, I have multiple devices I want to monitor behind a NAT of which I have no control over, so I have to do active checks.

Since I'm too lazy to deploy a proxy in that LAN, I figured it would be comfortable to have the option of skipping that and having each agent on each client communicate with a different port.

I have already tried that by issuing IP's in the format IP:PORT for those clients and there is certainly a connection being tried to be made by both the server and agent on that port.
I checked that with a netcat on that port and I am getting "ZBXD4{"request":"active checks","host":"test3"}"

The issue seems to come from the fact that Zabbix server listens in only on one port (by default 10051)

Please consider this as it will greatly increase the usability I can find in your project.