Edit 2: I just found out about the black bars add-on. It says that it will change the aspect ratio to 16:9 but will it also fetch the 16:9 thumbs?
The current implementation of the Black Bars Remover Add-On fetches the "mqdefault" thumbnail, e.g. https://img.youtube....c/mqdefault.jpg. These thumbnails are 320x180 (16:9). If you need to go larger, you could likely edit the source of the add-on to use "maxresdefault" instead of "mqdefault".
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