I figured that as a reference to Two-Face aka Harvey Dent. The only other famous Batman ally who turned evil is Jason Todd, but I don't think that happens yet in this new timeline.
I agree, Two-Face was one of the first things that came to my mind too. The point I was trying to make is I believe there's potential for a future Batman film exploring the history behind that reference, and have Bruce pit against a gang of villains who were good at one stage, and were people who Bruce had personally dealt with in the past. I don't think anything like that has ever happened on film or TV, except when Batman tried to help rehabilitate Two-Face in BTAS.
Imagine this - Batman had encountered troubled people like Waylon Jones, Harvey Dent, Selina Kyle, Kurt Langstrom, Victor Fries, and Pamela Isley at some point in time, and even went out of his way to help them in the best way he can. They eventually became the villains they are today, which broke Batman's faith in people, and in the future, these very same people join forces to try and get him. That way, we get an even deeper backstory behind Bruce's path to deconstruction in BvS, and he is forced to face his demons once more in the present day.
I doubt it will ever happen, but I think there is great potential for that kind of story.