Why don't you like Rarity's or Pinkie Pie's? Sweetie Belle is Rarity's sister, so it would seem natural for her to take on the element of Generosity. As for Pinkie Pie, well, Peachy Pie's name is the closest I could find for a match.
With Pinkie I'd say it's not as much about names as it is about filling the role, and somehow the similarity in names makes it kinda weird. Also I don't think the CMC have to take their older sisters elements I always pegged Sweetie as being the next Pinkie. In some ways its more about the personality types than the elements. You need a fun lover and a fashionista so I've always thought of either DT or Silver Spoon as Rarity's successor.
I can totally see her forming a new main 6 with the CMC and other fillies heres how I see it Twilight-Dinky Pinkie-Sweetie Belle Dash-Scootaloo Fluttershy-If anyone can think of someone contact me because I am stumped AJ-Apple Bloom Rarity-Diamond Tiara(she reformed at some point)