There are numerous situations where when discovery rules from a template are applied you may run into a host where a trigger may be valid for one host but not another. 

For example interfaces on a firewall. 

An organization may have an internal and an external firewall but still want to have a template with discovery rules to find all of the interfaces and set them with a base default severity.

However you may want to apply a higher severity to specific discovered interfaces.

Having the ability to override the severity of discovered interfaces or other triggers that were discovered would be an absolute treat. Currently if a trigger is discovered via a template there is no means to change the severity of a trigger on a case by case basis. This can cause confusion in a NOC when lets say the WAN interface on the firewall is down but zabbix is telling them that a medium alert has come in for an interface. When in reality the severity should be much higher. Or on the otherside they wind up wasting time on disaster level rated trigger only to find it to be a non important alarm.