Add "name" or "type" to the interface list (like existing "default") and allow choose in template items which "type" of interface to use.
For example if we have host with 2 zabbix agent interfaces - type "wan" and type "lan", then we can make templates with items monitoring server on wan, and items monitoring on lan.
Now we can make interface selection only on host level, what prevent us from enjoy one of the advantage of Zabbix - templates.