I'd for this option:
Charles never became King of Spain so his line will rule on Netherlands and Austria (plus Bohemia and Hungary if he marry Anne Jagellon or both she and her brother Louis die without heirs) while Ferdinand rule Spain (maybe HRE Maximilian die before Ferdinand of Aragon so the latter is able to persuade the Cortes of Castile and Aragon to recognize as his heir Ferdinand, who is the younger son of Juana but is fully Spanish, being born and raised there instead of Charles, who is a foreign and already Duke of Burgundy and the newly elected as Holy Roman Emperor)
Or for making Charles V give the Imperial Crown, Austria, Burgundy, Milan and Flanders to Philip and the rest to Ferdinand.