We have an overabundance of ret paladins, so I wouldn't ever suggest that we should have more as a twink bracket. But that's strictly speaking with a preference for class diversity. However, I will say this:
Mages, Paladins, and Rogues do exceptionally well in this bracket.
Priests, Shamans, Warriors, and Hunters are viable as well.
Druids and Warlocks do not perform as well as they should in this bracket.
Drewbearpig recently listed some percentages for the 49 bracket's twink composition:
49 Bracket
Paladin 16.5%
Rogue 14.5%
Shaman 13.0%
Hunter 11.9%
Priest 11.2%
Warrior 11.2%
Druid 8.1%
Mage 7.7%
Warlock 5.9%
Hope this helped!