(I don't know if this theory has been posted, I tried searching for a similar one but didn't find one. If there is post a link please!)
Adolin will become an enemy of the Knights Radiant.
My reasoning:
1) He kills Sadeas in cold blood at the end of WoR. This act dishonors him and his whole as a character, I think.
2) His brother is already a Radiant. This is just a thought, but I think it's very possible that there could be an internal struggle that will happen within Adolin in the next book. Thoughts like "Why is weak Renarin a Radiant while the best shardbearer isn't one?" These thoughts would lead him into joining Eshonai and the Voidbringers and being consumed in darkness.
3) The consequences of killing Sadeas. I don't know what it will be, but Dalinar won't stand for it when the truth is out.
I don't have much evidence, but I know there must be others thinking like me. It's just a strong feeling I have. Let me know your thoughts!