no idea what's up with the crescent hate, i find her one of the most interesting characters in the entire story. yes, at first you do want to hate her for some things she's done, but think about it more. is she really that bad?
speaking of crescent, if her team isnt a mono psychic, i have a feeling she has (or will have) a xerneas, unless an even more important character shows up. about the four shadows/lights thing, it goes without saying that this is related to how yveltal takes away life and xerneas does the opposite (at least if you've read those absol and gardevoir statues). madame x has an yveltal, so it would only make sense for a character with similar importance to have a xerneas.
also, moreover on the four shadows, i do agree with vince, i think ren is likely to be one of them. y'know, i didnt really like ren at the end of version 5, but after playing version 6 from the beginning, i find him a much more relate-able character. seriously, just try to put yourself in his position. i know he's done some foolish things, but imagine all of the pressure he must be under, especially after finding out about melia. plus, with reina being introduced, he's more likely than ever to develop in some way (likely in a similar way to how keta developed) and possibly even die.
but, that aside, the main reason why i think he's part of the four shadows is because madame x and her yveltal are likely related to them, thus it's extremely likely that the four shadows will be related in some way or another to team xen. much like keta, ren betrayed his friends thinking what he was doing was right, only to realize it really wasn't. he's not just going to keep serving team xen like that, he won't feel comfortable hurting melia. not just melia though, reina will probably act as a massive troublemaker and ren will have to risk something to save her at some point, something like his life (much like keta did).
as for the other two shadows, i dont know, they could be characters we havent even been introduced to yet, or they could even be characters that are extremely unlikely to become affiliated with team xen in any way. other than that, my guesses are mainly towards neved (we still haven't gotten proper backstory regarding who his daughter is) and geara (obviously related to narcissa in some way, id say he's her son due to the way they interact, so he could eventually turn out like keta or ren). madelis is easily the shittiest villain in the entire team xen, even eli/sharon are more deep than her. fucking version 6 and that moment with tesla, i cant take her seriously anymore after this. nastasia is a very important member, but i dont think she'll have a fate like this, though, i cant wait for the moment where we can finally punch her again, like we did in version 5.
as for the four lights, dunno, but nim (for being so mysterious) and melia (dont even need to explain why) are some good candidates. marianette is also a possible one, though she could also be the madame x counterpart i have referred to, who should possess xerneas.
also, speaking of madame x, i have a slight feeling that she will turn out to be a character that was among us at one point. i find no other reason to hide her behind a mask like that. i could be over-speculating, but man, am i thirsty for some more plot twists.