#1 Daniel Bryan

Honestly, there may only be one likely choice for Roman Reigns' tag team partner against the menacing duo of Luke Harper and Erick Rowan. While Daniel Bryan could still do much more as a heel, him tagging with Reigns to take on his former tag team partner, Rowan, along with Harper is probably the most logical choice. He was viciously attacked by Rowan a couple of weeks ago on SmackDown and while most thought it was a ruse to get one over on the Big Dog, that doesn't seem to be the case.
Bryan has unfinished business with the big bearded ginger and he might not need to necessarily turn fully babyface. He could be a tweener, somewhat similar to the role that Charlotte Flair is currently playing.
John Cena's last PPV's name revealed by Kurt Angle? More details HERE
Maybe this plot is more complex than any of us thought and Daniel Bryan is revealed to be the mastermind behind everything that has happened. Then Roman can enlist the help of some of the others on this list to try and take down Bryan's evil bearded empire. For the time being, however it seems like the Vegan Warrior could be Roman Reigns' initial partner against the reunited Bludgeon Brothers.
Whether or not Bryan then turns on Reigns afterward is a different story altogether