Furthermore there wasn't an available French or English alternative, which would have brought more international benefits.
Well, Marguerite of Angoulême's first husband was killed at Pavia. She'd be a reasonable possibility, I guess - only eight years older - 33 years old in 1525. Although the lack of any children would be concerning. Charles was intent on Francis marrying Eleanor instead, though.
There's also two Mary Tudors who potentially work. The elder married Charles Brandon immediately following her widowhood in OTL, but she'd otherwise be a good match. The younger Mary Tudor was, of course, engaged to him in OTL, but she's really rather young - You can't expect any children from her before the early 1530s, which is less than ideal.
BTW, why didn't Charles marry again following Isabella's death? A marriage to Mary Tudor (26 in 1542, when Henry allied with Charles again) to Francis I's daughter Marguerite (16 in 1539, 21 in 1544, to name the two most likely dates for a Valois/Habsburg marriage alliance), or to his niece Jeanne d'Albret (15 in 1544) would all seem potentially advantageous. Though Jeanne, with her claims to Navarre, would be an even better choice for Philip, who was more or less her age.