Because Cain v. Mir is already the #1 contender fight, short of some injury Snafu that never
happens in this sport. The winner of this fight won't be fighting the Loser.
Also, all of this analysis is null and void if Mir beats Cain. Though, that is the most unlikely thing to happen in my entire post.
I would not Advise the Loser of Cain v. Mir to fight the loser of Overeem v. Junior. Here's why:
If Overeem loses, and Cain fights Junior, you risk having Junior's first 2 defenses being Rematches(Overeem, and Cain), which kills the future HW title ambitions of both down the road, and stagnates the division Rich Franklin style, short of Cain beating Junior or Mir beating Overeem. Though the chances of one winning is probably above 50%, the potential risk of stagnation isn't worth it, in my opinion, from a business perspective. Seeing as how keeping those fighters (Overeem/Mir) seperate allows for two extra avenues of title contenders to emerge down the road through them being upset against a different opponent, while still maintaining the possibility of one putting together a win streak later and Challenging for their earned rematch in the future. In the HW division, whichever route allows for the most contenders to get up to Junior, i think is the wisest move.
If Junior loses, and Cain fights Overeem, The champion will then fight Junior for the title in their next match, as a rematch. And the same problem occurs as in the situation above, but the stagnation is even harder to avoid. If Overeem win's both, Junior is out of the title picture being 0-2 against Overeem. Cain is unlikely to lose against anyone below Junior, and at 0-1 Cain is likely to get rewarded a title shot against Overeem in a rematch. Same problem occurs. Stagnation and Cain/Junior Clean out the contenders while out of the title picture themselves. But even if Junior has to wait for his title rematch, He'll still clean out the contenders until he does.
The only 2 possible bright spots in this scenario is if Cain beats overeem and Loses to Junior, you set up a trilogy with Overeem v. Dos Santos and possibly a rematch/trilogy with Cain Overeem.
That and if Cain beats Overeem and beats Junior. Junior v. Cain get a trilogy, and Overeem can earn a rematch afterwords against Cain.
The problem with this convoluted outline is that, 2 particular fight endings in a row have to occur, to prevent stagnation, and that at HW i'd put at about around 20% chance to occur for If both times the winner is the favorite. Even worse if they aren't the rightful favorite in both fights.
So with all that mumbo jumbo out of the way, I think the loser of Overeem/Dos Santos and the loser of Mir/Cain is a reallllly bad idea.
So, my choices
Overeem is the loser:
Winner of Big Foot v. Big Country. But if Nelson wins he's probably still going to LHW. In which case i would defer to Shane Carwin. As i think the only other rational option would be the winner Cormier v. Barnett, and i would prefer that for Mir.
Junior is the loser:
Winner of Cormier v. Barnett. Or a Rematch with Werdum if he win's against Russow. If niether pans out, Mark Hunt all the way in a fireworks stand-up fight. Though i'd prefer to see Hunt go up against Carwin if Junior loses to Overeem.
Sorry for all the convoluted, baseless rambling. But thats how i would juggle the division in terms of Business, Entertainment, and my own personal preferences.