That reminds me of a story I read about Michelangelo. He stared at the lump of marble that was to become David for a very long time before making the first cut, when asked what he was doing he said "I'm working"
If he could permit himself to start over several times, I can too.
Is it possible to have a "clear but poorly-defined goal", or are you having a joke? "Fun" is an emergent property - recognizable, measurable but the creation isn't formalizable (though BTW your link mentioned something about game grammars...)
Poorly-defined, meaning: one man's fun is another man's boredom, it's hard to pin down what exactly "fun" is. Thankfully, though, there are many good educated guesses, so we can't act like "fun" is impossible to define.