We recently released a feature that does exactly what you describe:
This lets you add {{PLACHOLDERS}} and in the merge parameter you can do a find and replace. This will allow you to create templates with placeholders.
While we build out something ourselves, you are welcome to build out your own template gallery/marketplace. We welcome and encourage that.
We see this as a Wordpress/Bootstrap type model where third parties can sell templates/themes etc… that are compatible with Shotstack.