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Such launcher should provide context widget (i.e. menu), or icon with such widget, and this widget should contain list of application types, i.e. web browser, music player, text, editor, graphics editor, spreadsheet etc. When user choose one, then new icon of default application of selected type will appear in launcher, but from context widget of that icon, user should can be able to select other default app (i.e using radio button) - changing displayed icon (text/tooltip) and launch command, or (s)he should can launch from it other program, or can drag files to icon, and if he wait longer before drop, then context widget should be opened, allowing to drop to alternative application.

I'm sorry I didn't exactly understand your idea.

Do you mean that a program launcher instead has a list of program types, like "audio player", "web browser", "email client", and the user has the option to choose which application maps to which application type?

... and user have ability to add program type icon (sub)widget to launcher area, and to change default program for it from context widget (if other programs of that type are installed), can even add more icons of same type with different default app.
possibility to add uncategorized programs, or even to create categories will be useful.

your idea about kde-wide setting this manner default apps is also interesting, imho.

I'm not sure if I get your idea correctly, but I think what you wish is plasma-netbook as it is in KDE4.4.
Unfortunately I don't have any screenshots of it, neither did I found any...
So you'll will just have to take my word or install 4.4 yourself (I had it, but gone back because of usability issues).