What I would like to be developed is a Chromecast app that allows:
a) A slideshow based on a Picasa webalbum to be shown on my TV;
b) Simultanously (!!) music of my choice (e.g. from my Google Play Music library, or a free internet radio station, based on its url) to be played on my TV.
I would prefer an app where the choice of slideshow and the choice of music can be changed (other playlist, other station, other slideshow) independently (slideshow/music).
Such an app would allow people to use their Chromecast/TV combination as a digital picture frame and as a tuner/MP3-player at the same time.