Who do you think should be the sixth member? They have to be female, they have to be a unicorn, they got to be close to Twilight, and they have be part of the G4 franchise (FIM, EQG, Specials, Movies, Pony Life).
Sunset Shimmer and Trixie don't count. Sunset has cemented herself as part of the Rainbooms, and Human Trixie already has her own band.