Wait, why do you say that? I'd say Taln's extreme trauma from the Desolations and being tortured on Braize for four and a half millenia just might count as a "hole" in his soul. (Please tell me if this makes no sense. I didn't get enough sleep last night)
On another note, I think the discussion on Gavilar on the Gavilar theory thread has me extremely convinced that Gavilar will be Odium's champion. Why? Well, Gavilar is the perfect mix of a few character traits that Odium is probably going for in his champion.
Emotional drama - This one is pretty self explanatory. If, as he heavily implied in RoW, Dalinar is going to be his own champion, this would cause quite a bit of conflict in him with, y'know, Gavilar being his brother. This is where the El theory is lacking. Despite being extremely imposing and morally out of whack (is this a phrase?), he has gotten very little screen time, and Dalinar would have no hesitancy in killing a Fused whom he barely knows.
Strength - However, if Dalinar works out his moral dilemma about killing his brother, he also has to face him in combat. Gavilar wasn't as strong as Dalinar when he was fully alive, but who knows what he did in order to come back. Voidbinding? AnTiSuRgEbInDiNg?/??? Anyway, this is largely an unknown factor. Brandon could make Gavilar as strong as he needs to me to make it a fight that draws the readers' attention. This is where the "child champion" theory lacks substance. If Dalinar gets over his unwillingness to do what needs to be done, Odium would be screwed.
Evil Nature - Gavilar didn't seem that evil... up until the RoW prologue. There, we see him in his full awfulness, and know that this guy, he sucks. Bad. Anyway, this is pretty self explanatory. Odium would want someone hateful. Probably. This is probably the most questionable category, but it is my personal belief that the final battle should be against someone who's at least a little bit gray morally. This is where my Shallan theory fails. Shallan is a good person, despite her insistences that she isn't.
For more on why Gavilar is going to come back in the first place, see the Gavilar thread. I'm going to go read some Shadows of Self. Good night.