While we've added a ton of plugins (pretty much anything we think you might use) there might be something we missed, or another type of plugin you want to add. Using the Custom Plugin Template you can hard code any JS you would like or find an example online that you want to edit! Here is how you can add in the blank plugin:
Head to your promotions area and hit edit design
Under Add Layer, find plugins and then find Custom Plugin Template
Copy and paste in the code you found or start hard coding away!