In order to get most out of your Zabbix installation, it would be nice to have plugin support in the webinterface allowing people to write their own plugins and use it from Zabbix.
It should be able to insert itself into the menu and authentication system.

Additionally, it would be nice if actions/links could be added to the "popup" (if you click an item) function as well with parameters (e.g. host ID, hostname).

Functionality like this would allow people to use Zabbix for more than "just" a monitoring system
For instance by integrating it with their environment (e.g. Puppet, Saltstack or virtualization platforms like Vmware/KVM/EC2)