Over on his FaceBook and Twitter, Christopher Jones posted a picture of the old Super Friends Legion of Doom: Here, with the comments "I've gotten to draw at least four of these characters (different versions of course) in the pages of Young Justice, and three of them were my own designs!" and "Of course, three of them are also from issues that haven't come out yet..."
So, who are those? For the Super Friends impaired readers, from left to right: Riddler, Scarecrow, Sinestro, Solomon Grundy, Brainiac, Captain Cold, Giganta, Lex Luthor, Black Manta, Gorilla Grodd, Cheetah, Bizarro, Toyman.
And here's what we know: Grodd has appeared. Brainiac will appear. Since he's using past tense, it's a fair guess he means characters from #20-#25 (I don't think they're working on later issues). And since he stresses own designs, we can rule out characters that've appeared in the show.
Edit: as pointed out by Psypher below, Luthor is the one character Jones didn't design, he's confirmed in the solicit of #22. So we only have one left.
That leaves us with 7 possible characters for the last slot: Scarecrow, Sinestro, Solomon Grundy, Giganta, Cheetah, Bizarro and Toyman.
Who do you think? Who do you want?