[X] Carthago - The intended site of Angron's trial. Holding a council here would send the message that you believe the Imperium must hold itself to a higher standard.
To be honest it isn't Nikaea, hence the implied 'symmetry' is not as apparent, but is clear that Angron's trial on Carthago was intended to be this TL's version of that event, hence the foreshadowed events / original plans many points of view in the hypothetical novelization of this quest covered it extensively enough.
So it has to be Carthago, because there's no way the irony wouldn't be lost on the readers.