I gatherd some data and those are the heroes in the top 100 of CYL (I only listed the characters that are not in the game yet - gatekeeper since we will get him before the GHB)
So let´s analyse this. First of Yuri, Balthus, Constance, Hapi. I think Yuri is the most likely here. Yess he is popular but so was Felix and he did end up as TT+ unit. Having said this I still believe we will get a Ashen wolves banner instead. If Yuri does become the GHB there would only be 3 characters from Ashen wolves so… I think this will only happen if they decide to not do a Ashen wolves banner next (which might happen because I have this gut feeling that we might get Timeskip banners) Still unlikey.
Next would be Touma, Yashiro, Tiki and Meiko. Possible but unlikely. I mean we still miss TMS characters for harmonic which will happen for the next summer banner OR the dance banner. And please let it be the dance banner since this would be pefect. Getting only one of them as GHB would be a bit eh. <.< But that´s just how I feel.
Nyx. We still have yet to get another fates banner which will very very likely happen this or next month. And Nyx should be part of this banner.
This only leaves Pelleas, Volke, Boyd, Rhea and Manuela. Pelleas, Volke and Boyd could happen on a banner together but I am not sure. I still have the feeling we might see summer Boyd at some point. Reah is more or less in the game as duo (and spoiler). Not sure if she counts. I am not sure if Manuela could happen. Maybe? If we are talking about alts then Severa could also happen. What do you think?