I'm happy there will be a few more added members. My immediate first guess is still X-23 as one of them. Looking back at the history of the O5 there were a number of periods where it was essentially O5 plus one or two others and it worked out well. First the classic O5 + Havok and Polaris, and in X-Factor the O5 subbed Caliban in with him even being a participant in Apocalypse's inaugural storyarc.
Back when we got the first teaser cover, the images of the adult O5 there made me interested in having at least one of them thrown in the mix as well. My interest was helped by the appearance of two Angels flying around in IvX, lul.