With Catherine of Aragon, I maintain that the most likely match for her in a scenario where she cannot marry Henry VIII in 1509 is for her to marry the
Duke of Savoy. That way, Ferdinand gets a new alliance and Catherine gets something to rule over. However, there's a second option that's always been playing in my head: Ferdinand,
Duke of Calabria. Son of Frederick I,
King of Naples, he's only 3 years her junior and the Trastámara heir to Naples. OTL, he was a prisoner and later friend to the
King of Aragon, and then married his widow in 1526. But would it be possible for Ferdinand, perhaps after his only son by Germaine de Foix dies, to marry his single daughter to the
Duke of Calabria and at the barest minimum give them Naples to rule? From 1504 onwards, he did hold Naples in pretty much their entirety despite France's best efforts, and this way Catherine is able to remain a Queen. Here's how I see it going:
1509: Catherine is sent home to Aragon, into the household of her father and new stepmother, a woman 3 years her junior. Her stepmother has recently given birth to and lost a son. Catherine, at 24, is somewhat desperate for a husband and household of her own, the Infanta of Aragon angles for a match with a King allied with Aragon. Her aim: Vladislaus II, King of Hungary.
1510: Negotiations drag on with the King of Hungary, who isn't truly interested in remarrying and, with an heir and a daughter, doesn't really feel the pressing need for more children. Germaine de Foix continues to struggle to concieve after her first pregnancy. A plan begins to form in the King of Aragon's mind, to keep his line and family name in continuance.
1511: Catherine is formally rejected by the King of Hungary, and instead, her father crowns his prisoner and friend Ferdinand, Duke of Calabria as King of Naples and Sicily, as his forefather's had been, and later in the year, with the new King somewhat settled, he marries his daughter to him. Catherine is finally a Queen.
1512: Catherine delivers the first child of their union, a son named Ferdinand, Duke of Calabria. Healthy and large, the birth goes well and his grandfather and step-grandmother act as godparents for the child.
Now this isn't an exhaustive timeline, but I think it gets the point across. Catherine wasn't incapable of having healthy children, and I think in this scenario, in a better climate and with less politic pressures from her father (who isn't asking her to be his diplomat because he has a direct line of communication with the new King, who is essentially a vassal of his) she will be able to go forward in a much more stress free way.
The children I envisioned:
Catherine,
Infanta of Aragon (b.1485: d.1538) m. Arthur Tudor,
Prince of Wales (b.1486: d.1502), Ferdinand III,
King of Naples and Sicily (b.1488: d.1556) (b)
1b) Ferdinand, Duke of Calabria (b.1512)
2b) Frederick, Prince of Rossano (b.1514)
3b) Stillborn Son (c.1515)
4b) Maria, Princess of Naples and Sicily (b.1517)
5b) Juana, Princess of Naples and Sicily (b.1518: d.1519)
6b) Alfonso, Duke of Sant Angelo (b.1520)
As to England itself, I agree with everyone so far that it's James IV's game to lose. He's a King in his own right, married to the Tudor heiress and has a son on the way. He'll probably race to England without her, if just to get hold of Mary. As has been said before, Margaret Beaufort is still alive, and I could actually see her keeping her granddaughter out of trouble. Beaufort doesn't want her son's legacy tarnished with a civil war, so all she needs to do is make sure Margaret is able to be crowned before anything happens with the younger girl. Hell, having a new focus in the succession might actually give her some reason to live for a few more months, rather than OTL, where she saw the succession safe and was able drift into the pleasant goodnight.
If he has control of Mary, he can easily have Margaret brought to London, and despite what other's have said, I don't think the issue is going to be that Margaret is also a Queen. They're married, they have a child on the way and he has control of her heiress to marry if something happens to her in childbirth. James wasn't a madman, he isn't going to get pedantic on titles and honour, he's King in all ways that matter. Once little Arthur is born, and there's been a general agreement he'll live, His place in England is secured. All he needs to do is start ordering monogrammed towels.
For Mary, once the initial rush is complete, she's going to be pushed to young Charles of Burgundy as quickly as possible. I'd say in 1514, if not earlier, so that James and Margaret can get that alliance. She's not an heiress, but then again she's the most beautiful Princess in Europe and will come with a good dowry. Plus, it'll keep England from allying with France, particularly if the
Prince of Wales is betrothed to Catherine of Austria.
I see Margaret having quite a few children too, and I like the suggested lot of children already in this thread, but remember that Margaret's last child was in 1528/1529 OTL. While there's probably going to be longer pauses, I don't see James being the type to give her a break, and here, even if she's worn out, she'll probably have her last child around 1525/1526.
James IV,
King of Scotland (b.1473: d.1531) m. Margaret I,
Queen of England (b.1489: d.1541) (a)
1a) James Stewart, Duke of Rothesay (b.1507: d.1508)
2a) Stillborn Daughter (c.1508)
3a) Henry Stewart, Prince of Wales, Duke of Rothesay (b.1509)
4a) James Stewart, Duke of York and Ross (b.1512)
5a) Arthur Stewart, Duke of Clarence and Kintyre (b.1514: d.1515)
6a) Elizabeth Stewart, Princess of England and Scotland (b.1516)
7a) Margaret Stewart, Princess of England and Scotland (b.1518)
8a) Stillborn Son (c.1520)
9a) Arthur Stewart, Duke of Clarence and Kintyre (b.1522)
10a) Mary Stewart, Princess of England and Scotland (b.1526)