Thanks a lot for the links. It brings into perspective what those sites are missing,from what i can tell, that would make this idea more viable. But I do have to disagree with you on a point.
I do believe that a widget based game maker can make original games as long as its open enough to allow creativity. From a programming stand-point this directly affects usability, but I would approach it from a design position.
Imagine a standard 2d side-scrolling platform engine. Characters have a bunch of predefined states. Users must only choose which states they want their character to have(selectable through a survey or wizard). What differentiates Mario from Sonic is only a few actions difference.
They are mostly distinct in design. Allow users a "bone man" with pre-made animations and abilities. Users can build the look with our online editor. then the designs are applied to the appropriate section of the "bone man".
Background tiles and objects can also be skinned or selected from pre-made props.
Right now at my 9-5 I am working on a children's virtual world and the most time intensive process is asset creation. This would also be the case in the stated idea. Animations, character states, and npc's and props would have to be developed over time, and can also be user submitted to speed up the process.
I would do 2d games
a puzzle game (standard scrolling, adjustable rules)
a fighting game (actions ability and looks would be adjustable to name a few)
a side scroller action game
a racing game
and others.
All of these would have premade "bone man" animations stock that can define a player's personality and skill set.
The custom design is the key in my opinion. i personally would love this.