H8 dying earlier (usually in jousting accident) was done many times. Especially with him dying in 1536. But depending of part of that year we can have very different outcomes.
Why December (late December-I want Henry to have some nasty accident between Christmas and New Year)? Because of combination of events, that would not last long. Henry is married to Jane Seymour, but their son was not conceived yet. That leaves Mary as obvious heir (once it is clear, that her step-mom is not pregnant). Meanwhile James V is in France, about to marry Madaleine (news of uncle's death may not even reach him), Emperor Charles V is not yet widowed and Mary's OTL husband is 9 years old. That leaves... Duke of Beja?
So what happens? With whom Mary rules England?
Why December (late December-I want Henry to have some nasty accident between Christmas and New Year)? Because of combination of events, that would not last long. Henry is married to Jane Seymour, but their son was not conceived yet. That leaves Mary as obvious heir (once it is clear, that her step-mom is not pregnant). Meanwhile James V is in France, about to marry Madaleine (news of uncle's death may not even reach him), Emperor Charles V is not yet widowed and Mary's OTL husband is 9 years old. That leaves... Duke of Beja?
So what happens? With whom Mary rules England?