Imho, since they're all girls (except ezreal which I have my doubts) the new team should be:
Taric : to join ezreal
Sona : From all the support girls left its either Karma or Sona, and sona its been always a support pick that represents the role better than Karma who's been for long time a midlaner.
Tristana : could join to help her yordle friend Poppy
Caitlyn : She's the sheriff already so can be associated with guardian and protection roles.
Orianna : From all the girl mages left I think Orianna is the easiest to associate with protection and therefore I would nominate this one over Diana, Lissandra, Elise, Zyra, Cassiopeia, Leblanc and Anivia. Also Taliyah could be, but I think Orianna would be better.