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In slideshow mode allow user select as sources not only local filesystem folders, but everything(well, may be almost everything. At least something) that gwenview support: nepomuktags (show picture with this nepomuktag) , network folders, e.t.c.

Also integrating some photo hosting ( with public API ) like flickr, photo.net would be nice feature. For example "Slideshow all photo from photo.net with tag from author " e.t.c.

I think that a script system, like Amarok, would be a better idea. You can write your proper engine to access all you like.

Currently there are two plasmoids that can be developed using Picture Frame and a script engine.