I've long felt that Cantata was a poor choice of default music player due to its baffling first-run setup wizard. But VLC has its own issues, and is really not that well-optimized for being a full-time music player. IMHO a better solution might be to engage with the Cantata developer and request a mode that doesn't present that wizard, or presents a vastly simplified version of it. Failing that, we could package Cantata with a config file that pre-populates the requisite information so users don't see the wizard at all.
Also, you might wanna make it look consistent with the rest of the desktop (somehow).
One issue per ticket, please. See https://community.kde.org/Get_Involved/Bug_Reporting#One_issue_per_bug_report
Nevertheless, that's very pretty! We can't include software that isn't packaged, so if that's not available in package form in Ubuntu, you might want to consider opening a LaunchPad bug requesting that it be packaged, and if and when that's done, file a separate Kubuntu ticket requesting that it be installed as a "recommends" dependency for VLC in Kubuntu.
What about Amarok? It's recently been updated after what looks like a dormant period. It's just audio, but it's got a ton of features like fetching lyrics from the internet and linking to artist pages on Wikipedia. And it's under the KDE umbrella, which Cantata is not.
VLC originally stood for "Video LANClient", so I never think of it as a music player. It's an exceptional piece of work and comes close to being a perfect video player.
I think there's room for both and let the users choose!
Amarok isn't yet ported to version 5 of Qt and KDE Frameworks, so it can't be included until it is, unfortunately. Once that happens, I think that's a sensible choice, especially if we ship with a config file that presents a humane and user-friendly default view (I expect asking upstream to change the default view will be a can of worms). Alternatively, and since that might take a really long time, we should consider checking out Elisa and Babe for Kubuntu 18.10. They're both very slick-looking and developing quickly.
Seems cool and consistent with the guidelines.
Elisa is promising, but not ready yet--especially for an LTS. I took it for a spin two days ago for the whole day, and it's at best beta-quality. It's very promising and advancing quickly, but it needs some more time.