I could see him winning the WHC within the next 6 months and feuding for that. But as for WWE Title? Probably not until 2014.
Honestly, him being in the title picture so early will only make him succumb to the Goldberg syndrome. Push him as a monster with limited abilities and eventually you have him lose. Unless you have a controversial finish everytime, he'll eventually either get buried or fans will lose interest in him because we've seen this movie several times before and it never ended well.
Don't think he'll win the big one anytime soon at least until Mania. The feuds/storylines for the WWE Title from Royal Rumble to Mania is pretty much going to rotate around Rock, Punk, Cena and Lesnar.
I see him winning the WHC before the WWE Title. He has the strongest chance to win the Rumble this year (assuming there's no Brock) among the current superstars unless he's booked in a singles bout or something. Then aims and goes on to defeat the WHC at Mania.
Where that leaves Ziggler and his briefcase I don't know. Ryback's surely not going to drop the title after a month or two reign, plus I don't see him defeating Ziggler at Mania assuming he's WHC since they both need that big win over a household name to put them over.