I wanted to know if there is any reason why I can not put a 200 amp main lug panel in this situation. There is currently a 150 amp main lug panel in place with about five double taps. The main disconect 150 amp is outside next to the meter. this would give me another 10 spaces

Nothing wrong with it at all as long as the load being served isn't too much for the 150 amp main. An easier way might be to just add a sub panel next to the main panel.