Edward VI of England, Henry's much-desired son. He became king of England at age 9, but was an extremely sickly child, and died in 1553 at age 15. It's kind of ironic that, for all Henry's efforts to sire a son, his most celebrated successor was his neglected daughter Elizabeth. Had Edward recovered from his ailments, reached maturity, and come to rule in his own right, all of English history would look very different.
First of all (aside from Mary and Liz never coming to power), if Edward married and sired heirs of his own, then the Tudors would reign for more than another generation. James would not be invited to take the English throne; and the English succession would continue through normal male primogeniture. However, England and Scotland might still be united -- Edward was betrothed to the future Mary, Queen of Scots. So...Mary, Queen of Scots's son might still become King of England and Scotland, but he would not have been the King James of our timeline. And of course, since Liz never takes power, Mary, Queen of Scots will never claim the English throne, plot to assassinate Liz, or be imprisoned and executed by Liz.
Secondly -- Edward was raised a Protestant, like Liz and unlike Mary; and it was under Edward that the Church of England formally separated from the Papacy and became a Protestant church. Since he was a chronically-ill child, Edward probably personally didn't play a huge role in the English Reformation, but his reign would see a lot more Protestant trends in theology, like iconoclasm, denial of Purgatory, and denial of the intercession of saints. Thomas Cranmer, the Archbishop of Canterbury (and author of the first English Book of Common Prayer), was responsible for Edward's education; and Cranmer was heavily influenced by Martin Luther and especially Hyuldrich Zwingli. So it's likely that Anglicanism would have been more Zwinglian than IOTL. Also, perhaps ironic for someone who was so against the veneration of saints -- the Church of England today honours Cranmer as a martyr, as he was executed for heresy by Queen Mary. So if his protégé Edward ruled instead of Mary, that wouldn't have happened.
I don't know how this marriage between a very Protestant Edward and the very Catholic Mary, Queen of Scots would work. Certainly, the Pope would permit -- even insist -- on an annulment; and Edward would probably have the power to divorce or annul the marriage according to his own Church. However, although Mary was a Catholic, Scotland had a Reformation of its own, and Mary was fairly pragmatic. If she divorced Edward and championed Catholicism, there'd be a civil war in Scotland, and the Scottish Protestants would have the support of England. She might find herself exiled and disgraced, just like IOTL. IOTL she did stick with Catholicism -- but she was the wife of the King of France; and she did march against England (with the support of English Catholics), but in this timeline she'd be England's co-ruler or at least its queen-consort by going along with the Reformation. The motivations are completely different; and to me, Mary, Queen of Scots doesn't seem like she'd be a Catholic out of humble piety. I think she'd participate in the Reformation, and use it to her political advantage. And in the meantime, she and Edward would sire children, and the next monarch of united England and Scotland would be a Protestant of the Tudor line. Maybe they'd still name him James. King James VI and I, of the House of Tudor.