Besides his niece?
I mean, the guy succeeds to the highest secular rank in Christendom in 1657, and I could imagine that for a time, there were hopes that he'd marry Maria Teresa of Spain (OTL queen of France). That takes us through to 1660 when she marries Louis XIV.
Felipe IV didn't want to send Margarita Teresa to Austria, and by the time of his death in September 1665, her marital future was still undecided. Then bam! by December 1666 she's empress.
I'd imagine that after June 1665 Klaudia Felizitas starts looking tempting, since she's now an heiress herself, but before that, her uncle is alive and set to produce sons (which means I can't see Leopold saddling himself with what
@Valena called the mother-in-law from Hell unnecessarily).
So, was anyone else ever considered for Leopold I? And in the event that neither of his first two wives is available (Margarita cause she's married elsewhere, and Klaudia cause there's a male heir to Tyrol), COULD anyone else be considered? (Calling
@Vitruvius,
@isabella and
@VVD0D95