Happy to contribute :

You can't customize the app buttons unless it allows you to which is not the case with Spotify.

1/ display the same touch bar UI when the app is in the front or background otherwise it's confusing.

2/ keep the spotify controls visible when using chrome for example (chrome doesn't have any touch bar controls yet). Right now I have to click the newly added spotify shortcut all the time. Maybe a way to decide which app should be displayed when the space is unused would help (in the OS settings).

I like the song play progress bar, it would be even better with the current and next artist/song names and covers.