I heard that the legal matter with Wonder Girl was settled, so now their allowed to bring Wonder Girl into the Young Justice show. So I highly doubt they won't bring Wonder Girl into the show, but it'll likely happen later this season or maybe even next season. Plus aren't they gonna kill off John Zatara?
 
The real question is which Wonder Girl? Donna Troy or Cassie Sandsmark. Personally I perfer Cassie, it would be nice to see her and Connor hitting it off. But just because you bring in Cassie dosen't mean you should'ent bring Donna Troy, but I would perfer if Donna came more as a guest and not a regular. 
  
The only other person I can imagine other then Wonder Girl is likely Supergirl. Though I can imagine Supergirl might appear in another season of the show.

the version that would use is probably going to be donna troy, the robin is grayson, kid flash is wally,  speedy is roy not mia, it really, makes no sense for cassie to be  before donna.And I doubt we'll see Speedy  again except for the episode where he becomes Red Arrow.

Donna Troy 100%
 
Maybe Cassie could just be a guest appearence.

It is most likely to be Donna
 
@Nerdman3000:
Not exacly, since Donna is Dick and Wally's age. Well also see Tempest, who is Aqualad's best friend.

I want Zatanna!!

I just hope that Donna and Connor don't get Romanticly involved. My guess is that if Donna is introduced mid-season, and the show does reach at least a third season, then most likely in that third season Cassie will replace Donna. Till then were stuck with Connor and Miss Martian as a couple.

*cough*.....Static..... :)

Robin and Donna would be nice too.

hmmm Starfire?

Greg Weisman mentioned on his site that Bombshell was apart of the Masterlist to appear on the show, But that she most likely won't appear until season 2 once they get picked up for it. So that goes to show you there not just looking a the A lister sidekicks, there looking at everyone. So who knows who they'll get since its a new earth, some heroes may come sooner than later. Possibly Blue Beetle, Kid Devil, or  maybe even Beast Boy. As Greg also said before the scope is wide.

My guess as to who the 2 new YJ members will be and why. 
1) Wonder Girl, because a clip showed the 6 current mentors standing alongside Wonder Woman. Since the show is an early DCU, I assume they'd go with the Donna Troy version of the character.
2) Captain Marvel Junior, because he's a cool character, and because his two sides; one a magical powerhouse and  the other his normal, paraplegic self make for a character who understands the balance of power.

" My guess as to who the 2 new YJ members will be and why.  1) Wonder Girl, because a clip showed the 6 current mentors standing alongside Wonder Woman. Since the show is an early DCU, I assume they'd go with the Donna Troy version of the character. 2) Captain Marvel Junior, because he's a cool character, and because his two sides; one a magical powerhouse and  the other his normal, paraplegic self make for a character who understands the balance of power. "

Garth will be aqualad best friend

The 2 new team members are separate from the original 6, so Artemis is not 1 of them. And any new member is more than likely going to have a mentor in the JL. YJ might be all legacy characters, or characters associated with the JL members. 
 
Zatanna might be  a little old to be in YJ, since Zatara has some serious wrinkles if you freeze-frame him in episode 1 on the computer monitor.
 
Besides Wonder Girl and Captain Marvel Jr., I also see Kyle Rayner and Golden Eagle as likely candidates for YJ. And if not Zatanna, then maybe her cousin Zachary could be making an appearance.

Rayner was out of High School when he got the Ring plus he'd get sent to Oa for training first he wouldn't need YJ to learn to work as a team
 
Captain Marvel Jr.

zacharia should be added...he good be the naughty boy who swings from being bad to good to bad too good

Wonder Girl would be my first choice.  Since this is an animated universe, I think Donna would be the better one for the role.  The trifecta of Bats-Sups-WW needs to be completed in Robin-Superboy-Wonder Girl.  Megan is cool but she's doesn't properly fill the position.  I think they need to add another female teammate so Wonder Girl is the best suited for the position.  Now, since we're talking about a separate DCU where things are flexible, I'd like to see Arisia or Jade join the team to fill the Green Lantern spot. I know they are older in comics than the assumed ages of the characters in Young Justice, but I still believe a GL representative needs to be included.  I only mention those two because they have links to established GLs in the DCU.  If someone else has a better GL stand-in to suggest, I'd be open to that, too.

I'd love to see a Earth-based teenaged Green Lantern
I know none of them are (Kyle's the youngest I think) but I'd love to see Kyle as a teenager ^.^
-Virus

"The 2 new team members are separate from the original 6, so Artemis is not 1 of them. And any new member is more than likely going to have a mentor in the JL. YJ might be all legacy characters, or characters associated with the JL members.   Zatanna might be  a little old to be in YJ, since Zatara has some serious wrinkles if you freeze-frame him in episode 1 on the computer monitor.  Besides Wonder Girl and Captain Marvel Jr., I also see Kyle Rayner and Golden Eagle as likely candidates for YJ. And if not Zatanna, then maybe her cousin Zachary could be making an appearance. "

Remeber this is Dick and Wally's time...maybe Starfire, Cyborg, Beast Boy and Raven should appear

In relation to the comics, it should be Cassandra thats WG, but then again, they put Dick on the team as Robin instead of Tim. 
 
Who should join otherwise? 
Everyone who's not on the team so far.

I was actually thinking of Miss Martian as being the one to be killed/leave the team too. I really really like her, but then I thought that she "hides true appearance". What if they did a different depiction in show in which she is not a reformed white martian like in the comics, but she an actual spy/traitor instead. This is a different universe. Maybe Martian Manhunter really believes he is her uncle, but she is lying. It would make her character really intense especially with respect to her sweet-innocent persona.