Alternicity is obvious. What if
1. Prince Arthur had lived long enough to get Catherine pregnant? (To be sure, much would depend on this baby's character.)
2. Or at least until Henry had been betrothed?
3. Or on the other hand, he had died before the betrothal, so she was betrothed to Henry in the first place?
4. What if their son had lived? (Hard there, too, because his character would matter.)
4. What if Catherine had died of natural causes before he started to look elsewhere?
5. What if Henry had died of natural causes before he started to look elsewhere? Ideally, perhaps, just after he named Mary Princess of Wales? Catherine would have been able to act on her behalf; she was the one responsible for the victory at Flodden, with Henry on the Continent.
6. What if Mary had been married off younger? She would have been much more likely to have had a child.
But the historical fiction preoccupied me this time. If chiefly in a "this is why I write high fantasy" way.
For one thing, there's the historical constraints of trying to work around the known facts. There's no way to save Katherine Howard, or Ann Boleyn -- or Jane Seymour, either. This can create a strain on the most deft plotters. I remember once a work in which the hero was trying to assassinate King John. As you all know, he couldn't succeed. The writer, however, was so deft as to get him nearly in range of doing so when the famous loss of John's treasure in the tide, and his collapse, hit. I am entirely convinced that this, and the news of John's death, produced exactly the soul-shaking effect needed. It's the oddity of its doing so that makes it stick in memory. Most works have some signs of shoe-horning about the events. Much nicer to invent the events. They slide into place much more easily.
And then there's the characterization. No historical figure has enough detail to suffice as the main character of a novel, or even a major one. The author has to invent details, motives, events -- and often enough, sandpaper off enough to make it consistent. And the fun part is that historical fiction tends to clump about the same eras and so feature the same characters. All very well to remind yourself that one author's Eleanor of Aquitaine and William Marshall is no more related to another's than their characters named John Smith. But if you have one Ann Boleyn, beautiful, virtuous, filled with evangelical fervor, up against the corrupt, evil Catholic Queen Catherine, and one saintly Queen Catherine, more devout than any of the churchmen about her, being cruelly thrust aside for the scheming evil gold-digger Ann Boleyn, it's hard to forget that they are in one sense the same person -- above and beyond the actual characters that were actually there.