Grand Berserker options in descending order of likelihood:

A) THE MANDATED WAIFU (aka the obvious popularity farm): "Super Morgan" with her throne and all the Rhongomyniad lances on the field. Would be okay, but a very boring choice.

B) THE CLASS FACE (aka the obvious classic): "Super Heracles (B)" basically Megalodon but with Herc stance and moves. Pretty simple but still interesting enough but also very obvious.

C) THE LORE CURVEBALL (aka accurate but not marketable): "Berserker Tezcatlipoca", would probably be a Stand battle as with Musashi but with "true" Tez (the big skeleton from the NP) in the back. Should be the "correct" answer for lore, but as stated them trying to have a non-existing/class-swap Servant as the face would probably be unpopular.

D) THE BLACK HORSE (aka funny if true): "Super Cu Chulain (B)" basically the NP Berserker Cu but fully armored. The least likely of the still reasonable choices.

E) THE DUMB OPTIONS: Arjuna (B), Raikou, Berstoria or Beowulf.