It would be a nice feature in combination with hostgroup level macros: https://support.zabbix.com/browse/ZBXNEXT-1191

Beside support of User macros, I'd very appreciate support of LLD macros (in prototypes) as well.

trying to think of the implementation, the existing severity selector would turn into one of two radiobutton choices. the other would be an input field that would allow specifying user macrovariable name.
or even better - there would be an additional field. the selected severity would be used only if no macro was present. that way the interface would be simpler and a fallback could be configured easily.

there should probably be warnings both in the interface and in the server logfile when a severity macro would resolve to an invalid value (like "damn" or an invalid number)

A great use case for this is the possibility of you customize a script the value of the severity of the triggers for each type of item or alarm. This script generates the output in JSON and can be easily used in templates that use the LLD feature.

In this post (in Portuguese) there is a script and a real scenario that uses all. The only thing that can not automate is precisely the severity of the triggers.

http://zabbixone.com/?p=364

In this post, it is simulated a web application that manages events to be read by a script that generates the output in JSON to be monitored by Zabbix.