My thoughts on who is going to be the old favorite:
Dog: 1/2
Dog is a beloved character who, as long as you get, CANNOT LEAVE YOUR PARTY. Other characters leave on their own, or are able to be told to leave. Dog is not, AND there is upcoming DLC where you get another chance to get Dog. Dog is an easy candidate for inclusion, in my guess.
Leliana: 1/3
Leliana is good guess, because she's a talented character, with range and lock picking skills, plus she is romancable by both men and women, and is the "Statisistically likely" romanced character for male characters (Not a fact, but most people play as a hero statistically in role playing video games, and she is the heroic love interest, so while I am guessing here, it does have some inductive reasoning). She is well liked by a majority of fans, and in most of the endings, she stays with you.
Allistar: 1/2
Allistar is ALMOST a shoo-in, save for the... "role" he has at the end of most playthroughs. I highly doubt people will be allowing a person of Allistar's standing to go off on an adventure, but it is possible and he's seen a lot in the trailer. Still, there is a line that suggests he is not a party member, and if any character is on the sidelines form the old party, giving advice, it would make sense that it would be him, given the situation. This is kinda 50/50 here.
Morrigan: 0/1
Do to the nature of the ending, I do NOT think Morrigan would return in such an expansion. Her return will likely be much bigger than as just a party member. Her story is too big to be relegated to a side quest or side plot.
Sten: 1/20
Sten is possible, but unlikely, as he wishes to return to his people after the game is over.
Zevran: 1/30
Considering the many chances the player has of killing Zevran. I for one doubt Bioware will put too much work in reworking him for the sequel. It's a cost thing, considering how many people likely ended with him in the game.
Wynne: 1/20
Possible, but unlikely. MANY mages are seen in the trailer, and I doubt in her age, Wynne is going to become a Grey Warden, This is possible, but thematically, I just can't see it, especially compared to other characters.
Ogrhen: 1/4
I honestly would doubt this, but some people are adamant that they see Ogrhen in the trailer. As cool as he sometimes is, Ogrhen is not very high on the usage scale. Sure, everyone has their opinions, but I, nor the over 17 people I personally know who play this game (you can thank me later, Bioware, for getting 6 of them interested in the game myself) use Ogrhen. He is fun, sure, but compared to Allistar and Morrigan, he has no impact on the plot, other than being married to a character we intereact with (trying to avoid spoilers here). Also, compared to Leliana and Dog, he is not very popular to the boards here, as few people talk about him form what I;ve seen, Still, it would make sense, since Steve Blum, the VO, plays Irving the First Enchanter and all, that Bioware could get two characters to return, making it cost less moola to bring Ogrhen back.
Shale: 1/40
Doubtful. Shale is popular DLC, for sure, but the character was dropped from the original game, I doubt they will work hard to bring them back for the expansion. Plus, there are the countless people getting the game used or what not who will not have Shale. Still, in what could be called shady business, if you make it a requirement to have Shale to play the game, well, there's some money made for DLC, but some people will just plain not get the game in that case.
You Choose: 1/50
I like this idea, and there is some definitive logic behind it. It would make the game more impactful and emotionally... engrossing (seriously, if I hear that phrase one more time, I am going to punch a kid in his adam's apple. He will cry and be emotionally scarred, but I do not care), and it would make the replay value HUGE, I doubt Bioware will make the time and use the resources and spend the money for Voice Overs on SIX characters (not including Morrigan, who I believe will be absent, or Allistar, who I'm sure will be present, regardless, due to story reasons), just so one person can choose option A, while countless others choose another. Sure, it's a great idea, and there will be NO END to the amouont of complaining or arguing that would be avoided if this "choice" as some people suggest does indeed happen, but fiscally, I doubt it's feasible.
There, my opinon on the likelyhood of who will return. I vote for Dog and Allistar in the "probably" department, and Leliana as a good third possibility.
JUST MY THEORY, THIS IS NOT TO BE TAKEN AS GOSPEL....