Outros with animated backgrounds catch the eye of the viewer much easier than a static background. Moreover, you can also use the backgound to enhance the context of your end screen and explain what to see.
To use videos, you will need to have them uploaded to your youtube channel. You can also use one of the videos you already have uploaded.
These videos can be unlisted. So, if you have, let's say, an animated graphic background, you can upload it to YouTube as unlisted and when you create the template, select that video as a background.
Being unlisted means that other people and your subscribers will not be able to see it.
Your one requirement is that the video must be HD (1920x1080)
As soon as you are in the template editor, click on the "YouTube video" button to select your video
All your videos will fill the screen. Just select the one you want to use.
Select start and end of the video for your template. The process is the same as when selecting the videos for your outros.
Create your end screen elements as you usually do and save your new template
Your outro template is ready. You will notice that your template's thumbnail is animated. While the animation of the thumbnail is not in realtime, you can still see which templates have animated backgrounds.
You can find the example video template in the stock templates, when creating an outro. Try it out!