I get super attached to certain creatures really easily so it would be so awesome to be able to customize each ones lifespan (Maybe life edit it? So if you notice one you adore about to die you could add more days, or even perhaps an immortality option? On a toggle of course, not every critter needs to live forever - unless you really want to take conquering and island seriously)
It could open up really interesting story potential, because in sandbox it's generally less about surviving as a species and more about random fun with genetics and the personalities of your creatures, especially paired with other suggestions. With longer lifespans and sickness/immunity genes toggled off, you could have a King and a Queen being the only ones allowed to have children and forge the tribe out of the royal lines progeny. With longer lifespans and the ability for more than one tribe in a single game, you could have a pack of near-immortals and fleeting mere days of life in the other. Longer lifespans and the option to toggle immortality in a creature could give deities to lead your pack for your entire game. It would make for really interesting game play, and very customizable to what each player wants to do with it.