I've recently finished Prelude to Foundation last week and I clearly remember that R. Daneel Olivaw said something like:
"I have another plan in another planet. It is harder and more radical than Psychohistory."
in the penultimate chapter when talking about his mission in the universe in order to secure the position of humanity. Since I read the series before, I thought he might be talking about The Second Foundation (saying that, Psychohistory is also the main tool of The Second Foundation so they may not be separated, I am not sure), because mental powers really dominate the events in the galaxy during the following story-line and The Second Foundation is the first place that it is understood and used effectively.
But then in the Forward Foundation
the idea of a second foundation belong to initially Yugo Amaril, accepted by Hari Seldon and developed by Wanda Seldon later.
Whose idea was The Second Foundation?
An idea in Seldon's brain embedded by Daneel before he left him.
(or) Something that came to Amaril came all by sheer luck and Seldon found it logical
(or) Really an inevitable destiny pointed by Psychohistory so it was what Amaril saw first and Wanda proved with math later?
(or) A mistake of Asimov that when he first wrote Prelude to Foundation in 1988, he had introduced the Robot series already and referencing the idea of Foundation and Second Foundation by Daneel was a good idea to connect two universes. But later in pre-1993 when Forward The Foundation is written he thought a different way OR (unlikely) forgotten. It seemed better to credit the all Foundation plans to the main characters.
(or) Daneel was talking about something else which I cannot remember right now.