This has probably been Discussed ad Nauseum around here but what do you think of Izzy moonbow and sunny starscout possibly being a reincarnated Luna and Celestia?
Perhaps they decided to reincarnate voluntarily after growing tired of their immortality, wishing to live regular lives, vowing only that somehow they would meet each other again.
Perhaps they were Destroyed in an epic clash with a villain and Twilight cast the spell that reincarnated them.
I dunno. Izzy and sunny are new characters but two besties with a sun and moon theme seems convenient. It has to be fated.