The main six are really well known. But todays cadance is the main siblings. So for today pick one of the siblings from the mane six. Although scootaloo isn’t actually a sister she is close enough to most fans so feel free to use her.
So pick a sibling and take 30 minutes to draw them then 15 minutes to submit the art. After all the main six are really well known but their siblings all have their own life they live too.