Since just about everyone has been asking for the CMC to get their cutie marks, and it was part of Lauren's original vision for the show, I thought this would be a good topic to cover.
Here's my thoughts on the mater:
A pony named Gearhead comes to town, saying he's an old friend of AJ's parrents and wants to help her make Sweet Apple Acres great with mechines. Applejack is not cool with the idea, and it comes to a head with her competing against the mechine and hurting herself in the prossess. Applebloom realizes that AJ is being to stubborn and somepony has to tell her off (maybe with Luna's help?). Applebloom gets Gearhead to tell AJ how he became friends with their parrents, which gives AJ context to why Gearhead thinks the way he does and accept him. Applebloom realizes that she can get ponies to work together in a way no pony else can.
Sapphire Shores visits Rarity's shop and overhears Sweetie Belle singing. Seeing that Sweetie Belle has real tallent, she tries to help her shine. Rarity tries to explain that Sweetie has stage fright, but she won't listen. Sweetie Belle struggles to get away from Sapphire. Eventually, Fluttershy talks calmly to the two of them, convincing Sapphire that she overstepped her bounds and Sweetie that she needs to face her fears. You can tell where it goes from there.
Upset to be only foal left in her class with no cutie mark, Scootaloo visits Discord for comfort. Discord offers to take her on as an apprentice, but secretly tells Scoots parrents to let her stay over with him. He puts Scootaloo through a serries of "tests" that gradually reveal to her what her true tallent is.