Assume John Habsburg (19 October 1537-20 March 1538) son of emperor Charles V do not die as infant. How would Charles split his lands having two surviving sons instead of one? Spain for Philip is obvious, John would likely inherit Netherlands/Burgundy (so Philip's marriage to Mary Tudor is affected-he could not promise Nethetlands to their potential child if his brother is destined to get it). But the most interesting question is Imperial title-throne of HRE was elective in theory, but de facto hereditary in Habsburg dynasty. IOTL Charles wanted his only son to inherit his both Spanish and Imperial crowns, but electors didn't liked idea of another wandering emperor, so he abdicated in favor of his brother Ferdinand (who is also one of electors as Czech king), but he still hoped that Philip could get Imperial throne after Ferdinand's death ( is it possible that Charles could agree for HRE throne going to Ferdinand hoping for John to succeede him later). ITTL Charles would likely want his younger son as emperor, which should be acceptable for electors-John would be raised as Spaniard like his brother, but Ferdinand was also Spanish born, so if uncle assimilated into German culture over time nephew also can.
So who is more likely to get the throne of HRE aftet Charles?