His eldest daughter and after that can't have more children. She is married off not to the king of Poland but to her cousin Philip. They have one child inmidiatly a healthy boy that succeedes to the empire but after that keeps having girls. Elisabeth even outlives Philip so he can't remarry for more male heirs. What happens to the Hapsburg empire and Europe as a whole?
So the PoD is in summer or autumn of 1526. Hm, to keep things simple let us say that the battle of Mohacs and the death of Louis II. happen as in OTL.
Assuming that Anna dies in 1547 as in OTL, Ferdinand is then 44 years old. Maybe he marries again, but okay, that marriage remains childless.
But why would Elisabeth be married to Philipp? OTL, Ferdinand was elected as Charles heir in the HRE in 1531, and by then no one can know that he will have no sons. So assume the same happens here.
This marriage means a strong indication that Philipp will succeed Ferdinand as ruler of the HRE. I do not see why Ferdinand would simply agree to that.
The electors will no like that one bit, just as in OTL. They will greatly prefer that Ferdiand marries his daughter to someone who is bit weaker.
This scenario has written Epic Religious War all over it. Philipp would be unwilling to compromise even a bit with the protestant princes and probably try to crush them.