I think Cairne will help Baine, and Baine will fix things in Cataclysm. Or Cairne will come back and fix stuff.
They are both very wise men, and has never shined in the warcraft history. Is Cairnes death the only thing that happens to the Tauren? Seeing the Horde goes "crazy" with own wills etc I think it will get abit caotic. In all this, either Cairne or Baine will get in and solve alot of problems. Taurens time to shine.
Why do I think this?
In alot of books and other stuff, Taurens are said to be very wise. They believe in the nature, and want peace. Seeing Vol'jin, Sylvanas and Garrosh starts to follow their own wills, someone has to come in and fix stuff up. There are four alternatives;
1. Maybe all off those just come along and stuff will be good again, but I don't guess so, Blizzard is planning something bigger.
2. Thrall comes and save the Horde.
3. Cairne or Baine get their voice heard, and saves the Horde.
4. Lorthemar Theron get his voice heard, and saves the Horde. Tho I don't think so, cause he got nothing to do with all the changes.
With my theory, there are two alternatives left. And my guess is that they might be combined.
Thrall and Cairne has always been close friends. Thrall respects Cairne and the Taurens, and Cairne respects Thralls and his Orcs. Baine calls for his father's spirit, for help. Cairne as a wise man and friend to Thrall, tells him to seek up Thrall for help. Those two, combined, will slap the other leaders faces, and stop all egoism and fix the Horde.
Just a crazy theory I came up with, but it might happen. And would be cool imo.