Have you looked at YouTube Red yet?

So what is YouTube Red? Here is another add-on service for YouTube content that provides Ad-Free content for $9.99/month. It also gives you the ability to save the content off-line (stored on your device) so that you can watch, listen or play even when you don’t have internet access. It also let’s you play your content in the background so you can be using a different app then the player at the same time, even when the screen is turned off. You also get a Google Play Music Subscription at no additional cost if it is available in your country. However, this service does not extend to any paid services that you are currently subscribed to. You can cancel this subscription at any time.

For more information look here.