Based on what we've seen in the "trailer" for the cartoon series, and Hasbro's sneak preview figures traditionally being either semi-important background characters (Tion Medon, Zam Wessel), army builders (Episode II Clone, ROTS Wookiee), or characters we've already seen in one form or another (Jango, Grievous, R4-G9), I'm going to go out on a limb and say the "sneak" figure (if one actually occurs and it isn't a stupid poster instead) is two of those three. Namely, I think it will be an ARC Trooper with exclusive decoration differences from the currently shipping Obi-Wan/ARC comic-two pack.
My reasoning for this:
1-It's easy to do. Just do a couple of minor paint apps, take away the extensive battle damage of the comic two-pack version, and you have a relatively new figure.
2-It's an army builder, meaning Hasbro won't get stuck with a massive amount of backstock (GL in ST Disguise) that they have to get rid of elsewhere. And even if they do, they can still put it in a Battle Pack or in another round of "Saga Legends" assortments.
3-It's a "new" character that is integral to the series. Everyone will want one or more, and the visibility in the marketing campaign will make it even more desirable.
All this is just speculation on my part, but I think that out of all the characters we've seen thus far, the ARC Trooper makes the most sense. They'll probably save an accurately-scaled Grievous and/or IG-100 series MagnaGuard for the basic figure assortment, and new versions of the main characters will probably go that way as well. We're already seeing tons of AOTC-era Clone Troopers at retail currently, so those would be redundant. The only other character I could see as a mailaway that wouldn't be unspectacular would be this new Togruta female character we keep hearing about, but then again, if she's so prominent in the series, making her an exclusive mailaway figure would be like shooting themselves in the foot.