Might well. If he WAS poisoned as suspected. There's a good chance Caterina de Medici might be born male or her parents don't die which means she's less valuable for the duc d'Orléans (which would remove the potential poisoner - not saying she did or that it WAS poison, just what people thought...
I'd say much would depend on Thibaut's personality. If he's too ambitious (Carlos el Malo) he'll break on either Phil's France or Edward's Gascony and ¡hasta la vista! any lands in France.
Ideal type would be someone who doesn't make himself too problematic, either for Edward or...
Ooc: They were that OTL. Why does her marriage take place earlier here? Karl V obviously still succeeds as king of Spain (based off his and Claude's daughter's title/name), so what makes him let his sister marry earlier here?
Obviously Mary Tudor the elder wouldn't assert a restraining hand on her husband, Christian II of Denmark.
Ooc: the son of João III is a born when Katharina's pretty young (she'd only be 16yo), since she only married 2 years later OTL, what changed?
Fair enough. Would she name her son Carlos though? Mean, I know all traditional Iberian names (João/Juan, Fernando, Duarte, Luiz and Affonso/Alfonso, Henrique are taken, but Carlos seems a bit unlikely if Karl V DOESN'T become king of Spain, no?)
According to what I've read there were plans to engage her to Fernando VII (not the decade younger Molina) and earlier, a betrothal to Carlos Domingo (b.1780) with the intention by Spin of doing this if Louis XVI had no son.
But then Louis Joseph was born in 1781, Carlos Domingo died in 1783...
In 1576. Bella was either suspected by her husband of having an affair or actually was, and the guy had her strangled with a dogleash. A month later, Bella's brother, Pietro, murdered his Toledo wife in a similar fashion.