I think Mary Stuart, Queen of Scots was the only other serious Catholic contender.However the plan seems to have been to marry Elizabeth off to a catholic instead, making her a more suitable Queen. Mary Queen Of Scots continued to be a threat after Elizabeth's concession to the throne - hence her execution at Fotheringhay. As a daughter of Henry VIII, Elizabeth had the legitimate claim ( although some catholics refused to recognise the marriage of her mother Anne Boleyn to Henry VIII)