Ok, so this idea isn't very realistic, but I like the idea of more fur variation. Not simply in color, but in type.

Like there could be rex fur that's kind of curly, maybe a longer fur that's more woolly, and short fur like we've got now. Maybe they'd be wild-only traits to make NCL's interesting and fun again, since that's been suggested. Or maybe it would be hidden in the genetics, like the way Malteses throw out all sorts of different things.

EDIT:
Fur type ideas:
Tsavo (hairless or near hairless; limited manes for males)
Woolly
Short (just typical lion)
Rex (curly)

ANOTHER EDIT:
Another idea is to limit fur types to just different MANES rather than the whole lion. :D