This thread was shamelessly inspired by
@VVD0D95's question about Emperor Maximilian having a second surviving son, as well as my own
TL.
The Habsburgs were fortunate in that they
had enough crowns to go around in the scenario posited in the aforementioned scenario. But this was a good deal of luck on their part, since if Marie of Burgundy had had a brother, or one of the five coffins (Fernando II, the Prince of Asturias, Infanta Isabel, Miguel de la Paz and a "insane" mother) that Karl V walked past to climb onto the Spanish throne had remained empty for a normal lifespan, the Habsburgs would've been stuck hanging around in Germany/Austria/Hungary-Bohemia.
The scenario posited (with the help of a really strong fine mesh butterfly net) is as follows c. 1500 (all characters are OTL unless otherwise stated):
Maximilian marries Marie of Burgundy
Marie has a half-brother born of her father's marriage to Margaret of York, Charles II (b.1470).
Marie dies young (not necessarily in a horse-riding accident, but it's possible), leaving two kids.
Her son, Philipp (he might be Friedrich or some other suitably German name if he won't inherit the Low Countries), marries Infanta Juana of Castile.
Her daughter, Margarethe, marries Juan, prince de los Asturias/Charles VIII of France (whichever is more likely)
Juan of Aragon, prince de los Asturias, leaves a surviving son (if he dies on schedule) or lives longer than OTL
Charles II of Burgundy is married to some suitable wife (for my TL I posited Isabel of Aragon, OTL princess de los Asturias, but I'm open to suggestions) with at least
one surviving son.
Vladislaus of Hungary has the some difficulties having children with wives nos. 1 & 2, and marries some suitable French girl (Germaine or Anne de Foix). Has surviving issue with her.
Charles VIII of France is married to Anne of Brittany (if not to Margarethe of Austria).
So, we have c.1500:
Habsburgs in Austria
Trastamaras in Spain
Aviz in Portugal
Valois in France and Burgundy (separate lines)