I was thinking of all the things that can happen to cats naturally that could cause an early demise or issues. So I've got a few ideas I'd like to toss around. First off,issues that aren't visible/not very visible but will actually be unwanted. Because as it is now, everybody wants mutations, even if they're lethal. If there are some unwanted ones, maybe that'll change a little? Also some of these would be Such as:

Cataracts (Issue that can come along with older lions)
Deafness
Arthritis (Older lions)
Cancers (Any age, but more likely as they get old?)
Asthma (Cannot hunt, or breed as successfully)
Missing internal organs? (Lethal mutation)
Feline Diabetes

Also, diseases that can be spread. Line Feline distemper and feline aids

These would be issues that are more likely to be passed genetically, but not 100% to every cub. And they could show up in a seemingly healthy family lineage without a past of these sort of diseases.