One of the biggest use cases for downloading files is to download Youtube videos. KGet should be able to do this to remain relevant and used often by our users. It could achieve this by either adding youtube-dl as a dependency and using it as the download 'engine' to do this, or by adding the functionality itself. The former would be easier, as it more or less is just adding a GUI to select any relevant options. The latter removes the dependency. But either way, this functionality needs to be there for our users in 2018, as it is one of the biggest use cases for anyone downloading larger files.
Came here to create this same issue. KGet is pretty minimalistic right now. The de-facto tool for downloading online videos is youtube-dl, which is written in Python. So the direct way to achieve downloading of YouTube (& other site's) videos would be to write this as a KGet extension (that just uses youtube-dl in the backend.) If one of the developers could explain some more details on the extension, then I could try coding it up.