I'll keep it short because I'm not articulate or good at conveying my thoughts lol
The Brudos Interviews:
In Season 1, Episode 7, Bill and Holden interviewed Brudos for the first time. They conduct the interview, most of which is just Brudos fucking with them and denying that he was involved in any of the murders.
The interview concluded when Bill shows Brudos a photo: A victim of one of the murders being visibly photographed by Brudos due to his reflection in a mirror. Brudos effectively ends the interview after being confronted with this.
While Tench felt like they got nothing from the interview, Wendy had her own viewpoint. She found his constant denials just as interesting as Kempers overly eager openness about his own crimes. Wendy even suggests a new category when sorting types of serial killers: "post-conviction admission or non-admission of guilt."
In summary of their next interview, Holden gains a lot of favor with Brudos personally, but relations break down between Brudos and Bill. Bill will go on to refuse further contact with Brudos.
The Bundy Link:
During the third interview, Brudos is much more open with Holden, but still not admitting to his involvement in the murders. Eventually Holden is able to get Brudos to provide details about his own mindset and planning during the crimes by getting him to speak in the third person. Holden and Brudos theorize together the psychology of this theoretical murderer.
This directly correlates with how Bundy first started giving insights into his own mindset during his crimes. The interviewer got Bundy to speak in the third person when Bundy had previously not spoken about the crimes so deeply.
I'm almost certain that the next season would focus on this admission aspect, with Holden using this strategy to get Bundy talking in his own interview.
The Gacey Link:
This is fairly weak. We all know Gacey's face was up on their board and map.
Gacey infamously denied guilt throughout his life all the way up until his execution if I'm not mistaken. He even had some comically pathetic excuses in his denials. An interview with Gacey would most likely act as one to play off the Bundy interview and round out the new categorization.
Maybe I'm entirely wrong and it was already leaked or revealed what their plans were, but I just thought this was an interesting theory on where they could have been going with the third season.