Ferdinand I the future holy roman emperor dies in 1510. Does Charles v marry Anna of Hungary or his otl wife? Who rules the Austrian herditarry lands and Holy Roman Empire while Charles is in Spain or somewhere? What if Charles only has one legitament son like otl? What happens to the Hapsburg empire?
Well, as long as Maximilian lives (OTL until 1519) he will rule in Austria, obviously. I think that he will still be very interested in marrying his grandson (in this case Charles) to Anna of Hungary to secure the peace with the Hungarians. From the PoD in 1510 to 1517 Charles was educated in Brabant at the court of his Aunt Margaret. IOW he is not in a position to refuse. The marriage might even happen when Maximilian is still alive.
Now the childless early death of Louis II of Hungary and Bohemia is rather coincidental. He will be defeated by the Ottomans, probably, but not necessarily killed. Louis and his heirs residing in Prague with a secondary Hungarian court in Poszony might become the new normal of this TL.
One obvious result is that Philipp II no longer exists. I do not know whether the marriage treaty between the houses of Habsburg and Jagiello have a clause where the children are raised, but I doubt that rasing the eldest son in Spain is possible.
From 1520 onwards, Charles will be Emperor Charles V. of the HRE.
A TL writer is free to decide how many children of Charles and Anna there will be. OTL, Anna (and Ferdinand) had 15 kids, most of them surviving to adulthood.
Charles has certainly the right to divide his territories among his sons, but OTL he didn't, because he had but one legitimate son and heir.
BTW, in these times the hereditary Habsburg lands were traditionally divided in three parts: "Lower" Austria (the Archduchy, residence Vienna), "Inner" Austria (Styria, Carinthia, Carniola, Littoral, residence Graz) and "Upper" Austria (Tyrol and Further Austria with Württemberg, residence Innsbruck). For almost a decade, Ferdinand preferred Innsbruck over Vienna.
Basically, this is a PoD where the writer has lots of liberties because little is set in stone.