According to the Young justice episode guide on wikipedia and other websites there will be an episode titled "secrets".According to this artmis and zatanna will go out to the town on halloween night as they are stalked by a villian named harm with a terrifying "secret".Could this be hinting that Greta aka secret will make an appearence? Note:Harm was a villian in the young justice comic book series and his sister was secret. so this episode could either have her as a cameo appearence or introduce her into the series either has a member or a character that will stick around for a few episodes and not be seen again?

I've been hoping for this for over a year i was hoping charcoal girl in teen titans was greta but it was really solstice

I would count on her making an appearance........definitely
 
I do not think she will be joining though, as the producers seemed to indicate there would be two additions to the team in season 1, those two obviously being Artemis and Zatanna
 
Who knows what the second season will bring though