I’ve always believed Starlight Glimmer should have her very own Mane 6. She already befriended Trixie and Maud. Lightning Dust and Suri Polomare both have potential to be reformed. So with any luck, Starlight might convert them into embracing friendship. All they need is another pegasus to complete the set.
 
I also like the idea of the Human Starlight showing up at Crystal Prep and taking Twilight’s place as leader of the Shadow Six.
 
I would’ve like to count Gilda, but she’s not a pony.