I just saw a post that was somewhat similar to this theory, but there were a few differences and I wanted a place to flesh my theory out a little more fully than I would have felt appropriate to do in that post (the post is “theory about the end” and it’s quite good, I would recommend reading it). Please feel free to rip this theory to shreds.
I just finished my re-read of Oathbringer and it occurred to me that although Odium accepted to use a champion, he fled before either side had actually chosen a champion, as Dalinar is freaking awesome. My theory is that Dalinar is not going to appoint himself as the champion. It wouldn’t make any sense. He doesn’t have shards anymore, and the Stormfather told him that if he tried to use his Radiant powers in battle they would leave him. Dalinar even mentions several times that he is done being a soldier. So naturally, he’s going to appoint a 4th oath Kaladin to be the champion, because lets face it, Kaladin is the coolest and in a hand-to-hand fight he’s one of the best there is.
Odium is going to appoint Vyre as his champion, and will probably super power him with Yelig-Nar, and possibly Dai-Gonarthis as well (I say possibly because I really have no idea what Dai-Gonarthis is, nor what it can do). All these voidish powers are going to render Vyre completely unrecognizable to Kaladin. During their final showdown, right as Kaladin will have an opportunity to end things, he’s going to recognize that Vyre is Moash, really the only true friend that he’s had since Tien, and he won’t be able to go through with it, and he’ll hesitate. This will be all that Vyre needs, because by this point Moash will be completely gone, having given himself completely to Odium. Vyre will strike a killing blow.
At this point, Dalinar will be horrified, as he can’t directly intervene or it will break the oath that he made, probably freeing Odium. So instead, in a last act of desperation, he reaches into the Spiritual Realm and he restores Honor. This will of course take far more power than a human can use (as there have all ready been people saying that huge levels of stormlight are dangerous), and it will kill Dalinar.
But this will leave Kaladin open to take up the Honor shard, which I find only fitting as he was the first to bond an honorspren in this generation of Knights Radiant. With this new level of investiture, he’s going to be able to see how thoroughly Odium destroyed Moash, and it’s going to make him go crazy, and he’s going to attack Odium, especially since Odium is going to be weak from all the investiture that he poured into his champion to assure victory. Honor Kaladin is going to shatter Odium, then have a nice little chat with Dalinar in the Cognitive Realm before Dalinar passes on to the Spiritual Realm.
That’s going to be how part 1 ends. Part 2 will begin with Ba-Ado-Mishram (sorry about spelling) becoming a sliver of Odium, much as the Stormfather became a sliver of Honor, and then chaos will ensue.