Thank you all for your replies!! What do you think his reign as HRE might be like?
Let's assume he receives the S. Nederlands & Franche Comte as dowry From Spain (believing itself to be overburdened, Spain gives these territories as Maria Theresa's dowry with the same conditions as with Isabella Clara Eugenia. Spain decides to focus on Italy and it's overseas colonies and ignore the HRE for some time.)
How would this affect the campaign's of France & Louis XIV? (Would he redirect his energies towards the Spanish holdings in Italy or still look towards the Rhine, and how about the Imperial estates, since it's no longer the King of Spain but rather one of their own under threat would they be more willing to help defend these regions?)
Assuming he has a female heir (prior to his otl death in 1662), would Spain approve of her to wed one of Leopold I's sons (like Joseph I?, Looking at her age; she's a bit young for Leopold I himself unless she was born in 1659; and weds Leopold I right after the death of Claudia Felicitas.)
Assuming an OTL date for Josef's birth, she'd be closer in age to TTL Carlos II. However, if Leopold and Margarita's son, Ferdinand Wenzel, king of the Romans were to survive, then perhaps.
If Maria Theresa do not marry him, Louis XIV can not have legitimate reasons for attacking the Spanish lands. Anne of Austria, Louis's mother, was also a Spanish princess but was married in a double wedding (Louis XIII of France married Anne and Philip III of Spain married Elisabeth/Isabella of France) and so her renounce to the rights on the Spanish lands do not needed to be compensated by a large dowry (and Anne's was fully paid and she and her sibling shared both parents so that take away all of Louis' excuses for invading the spanish Netherlands). If for some strange reason Anne's children will ever by considered for the Spanish crown (on which they have bno valid right) will surely be after all the children of both her sibling: her brother Philip IV King of Spain (father of King Charles II, Empress Maria Theresa and Empress Margaret Theresa) but elso her younger sister Empress Maria Anna who never renounced to her rights (mother of Emperor Leopold I and Queen Mariana of Spain), so you will have this kind of line of succession to Philip IV:
1) Carlos II, King of Spain
2) Empress Maria Theresa and her heirs
3) Empress Margaret Theresa and her heirs
4) Emperor Leopold I and his heirs
5) Queen Mariana of Spain (her heirs are line 1 and 3)
[excluded] The heirs of Queen Anne of France aka Louis XIV and the Duke of Orleans and their heirs
going down on the line:
6) The heirs of Infanta Catherine Michelle, Duchess of Savoy (daughter of King Philip II of Spain)
7) The heirs of Empress Maria (daughter of HRE Charles V and wife of HRE Maximilian II)
8) The heirs of Emperor Ferdinand I
9) the heirs of Christine of Denmark, duchess of Lorraine (niece of Emperors Charles V and Ferdinand I)
and that is the full list of the heirs of Joanna of Castile and Aragon and Philip of Burgundy and Austria.
You see the difference with OTL?
About an evenbtual match for an heiress of the Netherlands: If Ferdinand Wenzel survive for some years or one of Claudia Felicitas's children is a son who live some years Leopold will be willing to wait a year or two for her. Another candidte can be his half-brother Charles Joseph, if survive to his OTL death. Is unlikely Mariana will marry her son in the family (OTL Spain refuted a match between Charles II and Archduchess Maria Antonia). But I think is more fun having a son from this wedding (who will likely marry Archduchess Maria Antonia as his first wife) who will help to secure the continuity of the Habsburg dynasty...