The Endless Sea also has its laws, and the entire Fantasy Dimension follows its own set of rules. The part of the sea area unveiled by the live broadcasting room is the deciphering of these laws and regulations, just like decoding a Plot World.
The future of the Main God Space is to uncover and interpret as much of it as possible and then grow and expand step by step, ultimately obtaining a vast and authentic "Immortal Realm".
However, a singular world is incomplete. With positives come negatives, with light comes darkness. To make a world vibrant and exciting, there must be enough variables to allow for its endless evolution. Thus, as the Main God, Li Xu did not seek absolute control over Naraku. Doing so would only create an avatar of himself, a constant, rather than variables and possibilities.
As for Naraku getting out of control, such a thing is absolutely impossible. Remember, Main God is just an identity and authority code for Li Xu, and not his entirety.