Remember how Applejack alluded to being the last in her class to get her cutie mark, and that it was the same for Big Mac? I think it's actually
because they're part of the Apple Clan. Just because you're born into a farming family doesn't mean you automatically
know that's what you want to do for the rest of your life, after all. Sure, Applejack's devoted to apples and apple accessories
now, but I wouldn't be surprised to learn she went through a phase where she didn't want anything to do with apples.
...Of course, given my luck, that'll be
Jossed in "The Cutie Mark Chronicles", but... eh.
I think it'd be cute if Sweetie Belle got a mark similar to her sister's. A crystal bell or note, perhaps? With Scootaloo, it's so hard for me to picture anything, for some reason...