We do not know what her plan was.
She was clearly not completely brainwashed by the High Sparrow as evidence when she gave Olenna the rose drawing.
It is also possible that she did not have a plan and was just trying to tell her grandmother that it was not safe to stay as Margaery might have known the High Sparrow was going to try to go after Olenna as well.
In all reality, whatever her plans were, they do not matter now;
Seeing as everyone is dead...