In OTL, Anne Boleyn had a third pregancy (after giving birth to Elizabeth, and after having a miscarriage in 1534) which resulted in the still birth of a four month old definitely male child in 1535. What if this had not been a still birth, and she had been able to produce the male heir? Arguably, Anne Boleyn was the wife with whom Henry VIII was most happy with. And frankly, if Anne Boleyn had successfully given birth to that damnes male heir that Henry was so looking for, England would have been spared of a lot of misery.
Also, what would be the long term butterflies from the continuing rule of the Tudors?