When Juan died Margret was pregnant with a girl, but tragically miscarried. If she is born alive, and survives infancy I see her marriage options being down between Miguel de La Paz and one of the Trastamaras of Naples, probably Ferdinand Duke of Calabria. Historically Ferdinand screwed them over in 1501 and took the Kingdom for himself. But they are still Transtamaras, so the Catholic monarchs might prefer them to inherit than to turn the country over the the House of Aviz. On the other hand Miguel was raised with his grandparents, and Isabella seemed especially attached to the boy.