An adult version of Sweetie Belle, according to my headcanon :)
 
As she became an adult, she further developed her time into learning how to sing along with spending time with the Cutie Mark Crusaders. She worked at Twilight Sparkle’s school of friendship, where she helped to teach magic to the students, along with helping out at the CMC camp during the summer.
 
Eventually Sweetie would end up becoming a protégée of Coloratura, who helped her with her singing. The two formed a close relationship with each other, and both Sweetie and Rarity consider Rara to be an honorary aunt of Sweetie Belle.
 
Sweetie Belle is now a famous singer across Equestria, with her specialising in singing pop music :)
 
She is bisexual, and is in a relationship with Babs Seed.