Now item prototype key format looks like:

function[parameter, {#KEY}]

This means that only items with parameters can be created from prototypes.
Furthermore in prototype key selection popup Zabbix suggests items where parametes are not supposed at all (for. ex. agent.ping, agent.hostname, external check), so they can never be created from prototypes.

Possible solutions:
1. Allow using lld macros in key function name, not only in parameters.
2. Remove pointless items from suggestion.