Why not starting smoothly by integrating div+iframe ?
This solution would involve only the modification of index.jsp + many of the css inside /style (the themes, mostly)
This post is a duplicate to another one, but as hakko said, people are free to help the project, so I'd say I'm motivated to modify it this way, as a starting point. I got some skills in js/html/php/jquery/css
About HTML5, Subsonic integrates a version 5.10 of flash jwplayer, which I replaced by the version 6 with no issue. This version can also be embeded in JS for an even better integration of videos, but that's not what I'm interested in for Subsonic.
[off topic] From my point of view, a video media player cannot be a music media player. Handle of massive music files organized by albums/artists/genres/title and so is far different from a video collection which has not the same needs. Also, videos cannot be organized like that because of the lack of an identical tag system accross the formats, as for mp3/m4a[/off topic]
Also, I would like to see in the frontend a 3 column pane showing [artist|album|genre] to select songs the way of the very old version of iTunes or almost like in the media library of Winamp... something like that :

(abandoned streampad web ui)
would it be possible ?