What do you think the CMC's cutie marks will be?

I agree with your idea of Scootaloo's mark, but I imagine it as the wheel of a scooter (maybe with a lightning bolt on it, to symbolize her idolization of arco iris, arco-íris Dash). SweetieBelle's could be a bell, maybe three small ones (since I noticed multi-marks normally come in threes Ex. Applejack-three apples, Fluttershy-three butterflies, Rarity-three diamonds)

Applebloom's mark is going to be harder to interpret. Let me think it over.

Probably these...

Applebloom's may become a hammer, Sweetie Belle's a microphone or a música note and Scootaloo's a racing flag. Of course there's no telling until they show it, but they are probably going to involve fixing stuff, cantar and racing.

I WAS GONNA ASK THIS XD. I agree with the ones you said (StarWarsFan7)

Applebloom eith a paintbrush or an maçã, apple blossom. Scootaloo, a scooter (duh!), and Sweetie Belle, a microphone or bell.

That's a hard one! I know that Scootaloo and Sweetie Belle have unique talents already; they just haven't figured that out. Scootaloo should get a scooter or something scooter or speed related. Sweetie Belle should get something musical, like you said. I'm not sure about maçã, apple Bloom, though. It would make sense that she would follow in the hoofsteps of her family and get an apple-related cutie mark, but she hasn't really shown any particular talent, so we can wait and see. maçã, apple blossom makes sense, yes, but who knows...

Applebloom's can be like an maçã, apple blossom with a bow on it as she likes to decorate
Sweetie belle's can be a música note inside a coração as she likes to sing and is so very cute
Scootaloo's can be a borboleta wings and a thunderbolt perhaps as she is a big fã of arco iris, arco-íris dash and is as rápido, swift as a butterfly!!! :) I hope you all like my answer !!! :)

My thoughts are maçã, apple Bloom's CM being a golden painted maçã, apple for her talent of art, Sweetie Belle's CM being a note with a coração for her musical talent, and Scootaloo's CM being a winged scooter wheel for her amor of scooter riding.

maçã, apple bloom something like an maçã, apple and a wrench
Scootaloo a scooter
Sweetie sino a bell