I'm really leaning toward an Aries vs Bully Ray match and here's why.
I think Bully is going to turn out to be a part of or run Aces and Eights. Correct me if I'm wrong but did A&8's ever REALLY attack Bully? I know they chased him around, might've punched him, but even if they did that doesn't mean he's out of the equation. I remember Immortal guys used to beat eachother up regularly before they teamed up. It can easily be explained. "They had it easy on me." or some Rayism.
That being said, if Bully wins the BFG series and then the reveal of the group's members and intentions takes place in the build up for BFG, Bully being behind it all and trying to weaken Aries along the way (smashed arm, knocked the fuck out last Thursday), I can not only see a Bully vs Aries match but also a reign for Bully with Aces and 8's behind him, protecting his belt from Aries.
While this may be thought of as bad for Aries, it actually is the best thing that could happen to him. Belt or no belt, as long as Aries is at the top and in the spotlight he's good. He needs attention and as any good hero, needs a nemesis. A heel that is just as good as him, persona wise, just evil. Aries is great at having the belt and chasing it would be even better. There are shades of revenge in the whole thing and that's always a nice plot for faces.
Bully gets to be Bully and hold the strap as the magnificent heel that he is and Aries gets to grow even more as a babyface by battling with the top heel in the company and, of course, eventually dethroning him as well as eliminating Aces and 8's with the help of the response team Hogan and Sting form.
This could quite frankly work with any of the BFG series contenders, including AJ Styles (and boy would that be a great feud), and I do think whoever wins the BFG series will be closely involved with Aces and 8's. They showed up either a bit prior to or right when the BFG series started and they slowly focused their attention on the world champion. They are working toward a goal. There's a reason why they're targetting him. Someone wants his belt. Currently the only people who can get it are the BFG contenders. It's one of them, in my opinion.
I am also not ruling out the possibility that we may NEVER see Aces and 8's faces. What if they're just masked goons meant to cause chaos and do Bully's (for instance) bidding. You know, like mercs. They don't show their faces, only the leader does and maybe after he's got what he wanted their services won't be needed anymore. OR, if you want to get extra Hollywoodish, have him use them to win the belt, hold it for long because of them and finally have them turn on him due to the good guys paying them more money to go away. Mercs don't have morals, they work for whoever pays. You take away Bully's defense (his goons), then you let Aries swoop in and take his belt back.
But, ideally, Bully should be "that" guy since I believe he has the skills to really take this to a level that none of the other contenders would, as heels.