Although a plan had been made, the selection of the target remained uncertain even now.
Because those involved in this plan were not one person, but rather more than a dozen Star Lords.
Each of these Star Lords followed a completely different path of cultivation; not every Great Ancient Emperor who was subdued could have their foundation be used by a Star Lord, and their Great Tao might not necessarily inspire all the Star Lords.
To commit the forbidden act of rebelling and slaying the immortals was inherently taboo; they could do it once or twice, but it was not possible to continue indefinitely.
Not to mention the potential for a True Immortal to intervene, but even without that, such actions, if expected, would lose the element of surprise. Should the Great Ancient Emperors become sufficiently guarded, the plan would not go smoothly and complications would arise.