This idea is kind of between snippets and tab history.

USE CASE
I frequently need to diagnose issues with our data. This typically involves using on of a few dozen queries. Currently each time I need one of these I have to 1) check to see if I have a recent version in tab history; 2) load it from some .sql file I happened to have saved it; or 3) rewrite it from scratch. For 1 and 2 I then have to search for all of the places where I need to replace WHERE expression values with data specific to the current problem.

SUGGESTION
Ideally there is a toolbar button alongside [Tab History] that brings up a UI like the Tab History window to select from a list of saved SQL queries. Double clicking on an entry would bring up a second dialog. If there are defined, named parameters in the query then there is a entry field for each value. Clicking OK causes a new tab to be opened with the contents of the selected SQL file, with the named parameters replaced with the entered values.

This is obviously not a minor feature, and I am certain there are a lot of technical and UI details that would need to be worked out to make it as simple and powerful as your other tools, but I think this would be a great addition to SQL Prompt.