For #GNOME app folks: IMHO we need to think about what to do with Totem / Gnome Videos. It has not yet been ported to #gtk4, which is increasingly becoming an issue.
Apart from not fitting nicely UI wise, it prevents us from using the newly introduced #wayland hardware offloading (zero-copy playback) and, crucially, from (properly) supporting HDR content going forward.
I.e. we either need a port - or should consider making an alternative a core app to focus on.
Short 
If we go for an alternative, the main requirement is surely #gtk4 / #libadwaita with adaptive UI. For distros it's probably also important to have some kind of dependency management to install missing codecs.
The language could probably be anything from c, rust, vala or gjs.
I wonder about the backend - should sticking with #GStreamer be a requirement or is something based on #ffmpeg / #mpv ok as well?
@slomo@toot.cat@rmader Something I forgot to mention in my other reply. While something based on mpv (or even just another UI for VLC ?) would likely work well functionality-wise, you'll have a hard time getting this shipped by RHEL/Fedora/Ubuntu at least in a way that is actually useful for users.
Thanks to software patents in the US, you'll either have them not ship the application at all or an effectively useless version that only supports free codecs. And then people will just get VLC from some other place anyway so they can actually play their movies.
With GStreamer these other codecs could be provided separate from the application, and there's even infrastructure available in GStreamer to make discovery and installation of those easier (although especially RHEL/Fedora go out of their way to make this as useless as possible too, to appease their lawyers).
@slomo@rmader Also to be clear, this is still a big problem with a GStreamer-based player and one of the many reasons why nobody uses totem.
User wants to play a video, video does not play, no information why or how to solve that, first Google result tells them to install VLC, problem solved.
Until that is addressed, any GNOME video player can be as nice as possible, most people are not going to use it. And addressing that is apparently a very hard problem because of legal reasons, especially if you're RH.
What would be needed is that after "video does not play" above, the user just has to click a button to make it play and that's also the experience you get on Debian, for example. Anything harder that would involve searching something on the Internet means that most likely the user is going to switch to something else.
@slomo@rmader That's an extra step though, and at that point you will already have lost many people towards VLC.
It's the same with totem from flathub FWIW (minus useful hardware-decoding support).
@slomo@rmader See also https://gitlab.gnome.org/GNOME/totem/-/merge_requests/356
> The only way to distribute totem we support is through Flathub's
Flatpak
and
> the process was always very dependent on end-users having prior knowledge of where to get codecs