You are starting from a misunderstanding.
From one point of view, you can say that every truthfully intended statement about the future is a prediction. From that standpoint, there is no distinction between predictions and statements about the future, no matter what tense is used. The future is unknowable.
However, most of us do distinguish between predictions and statements about the future. In effect, we distinguish between predictions for which we have overwhelming confidence and are treated as certainties and those for which we have less confidence and are not treated as certainties. A woman saying
I will be married tomorrow afternoon at four o’clock
is literally making a falsifiable prediction. She may be killed by a falling meteor in the next ten minutes. Her intended husband may change his mind and flee to Botswana. What is meant is that she believes that nothing remotely likely will falsify that prediction.
“Shall” and “will” may be used to signify predictions that are treated as virtual certainties or to signify predictions that are recognized to have some meaningful probability of being falsified.
The weatherman says there will be snow before midnight.
But it is common experience that the weatherman is sometimes wrong.
In short, “will” and “shall” can be used to indicate futurity as well as prediction. And “will” can also be used to indicate intention just as “shall” can be used to indicate obligation. Modals have multiple and somewhat overlapping fields of meaning, and even native speakers may not take full advantage of the subtle shades of meaning that modals permit.
