Hi. Just had a 4* Mr. Fantastic drop as a favourite but my only marked favourite is 3* Dr. Strange. Odd.
Bonus Heroes show up when you get a hero of that rarity from a non-vault token. If you get a Legendary from a heroic token or from the Latest/Classic Legends, you’ll get a Legendary bonus hero if the bonus happens. Same for Epic heroes. If you haven’t selected a Legendary or an Epic, the bonus will be a random Legendary or Epic.
I suggest choosing one of each rarity to focus on.