Please create an item that will return linked templates and a host group in which the host group is in a JSON format.
Similar as existing internal item zabbix[host,discovery,interfaces]
    
    

Second thing is to add to trigger actions some of the functions from autoregistration actions and network discovery. 
        Like
            Add/Remove Templates
            Add/Remove Hostgroups
            Enable/Disable Hosts

Use case: Load data from external API, that contains info about what host groups and templates should be linked to the host and compare it with results form such item.
    If there are differences, it can fire a trigger.
    As a next step trigger actions could add templates, remove templates etc.
Another use case:
    Create host and link one generic template to it, that checks sysOID. Then based on OID restrieved fire a trigger and based on that execute action to unlink generic template and link a specific one.