an autocomplete with hints for trigger expressions would be nice.
when writing an expression, sort of dropdown hints could show:

first it suggests to start with open curly brace;
after that, it shows that host or template name should be entered.
as it is entered, dropdown hint shows (limited) hosts or templates whose names match beginning of the entered string.
entries there can be selected with keyboard or mouse
once a matching host or template string is reached, it suggests that it can be continued or a colon can be inserted next.
then similar process to host or template name allows to complete item key (only matching existing keys on that host or template, of course)
once some complete key has been reached, suggest to continue with a dot, then do autocomplete/dropdown for trigger functions.
then suggest to continue with opening parenthesis and show what params are available for the function, which one we are entering right now etc