sizida, there is no 'early planning', it may seem like it was all put together and thought of before the series was even made, but it's not.
99% of all the great secrets and twists and pre-cursor events and secret sub-plots of gaming series are thought up while the game is still in production, you may think that they have all the mysteries thought out already and all the hidden secrets and plots planned out, but they don't, the majority of the time they make all the connections and tie up all the mysteries while the game is still being made.
And it's not all done by 'the creator' because 'the creator' also happens to have a team of people working with him and a team of writing staff and story-board writers and consultants for his, and his teams, ideas