If you decide to build a mansion you should adopt the classic pattern. In general there are only two floors for a mansion. You could separate both floors in two half's so you could organise better when it comes to the drawing board.
On the first floor you should definitely have a big porch where you can relax on those beautiful evenings watching the sunset. In this part of the house you could also have a quest quarters because it's a fact that friends and relative will visit you and they will need a nice place to spend the night. If you are a book lover you could assign a room for a library in the second half of the first floor. It's custom to have the kitchen and pantries also on the 1st floor so be sure you have enough room for them. The dining room can be close to the kitchen because it's practical and the servants won't have to cross al the house to serve you the food.
On the second floor the first half should be designated to the master bedroom and the secondary ones with 2 baths closer to them. If you have a private cook or other permanent employees you should consider making a few more bedrooms in the second half of the second floor so they have a place to stay. These two arias should be separated so both yours and their privacy is respected.
These floor plans of mansions are a general pattern but if you stick to this basic ideas we are sure you'll have the great mansion you always wanted.