You deck does not have to be square or rectangular in shape. For example, a 45-degree angle at a corner can break up areas with long, straight walls or other features. Trees can be surrounded by contoured decking, or garden architecture can be put on plinths built in to a reinforced deck.
The height of the deck can also be variable. A raised decking area would help to maximise a panoramic view from your garden, or will add depth and dimension to a formal garden.
On sloped ground you could build a stepped or terraced deck with slight variation between levels, or with a little more groundwork, you could build a level decking area accessed by a matching oak staircase.
Spas or hot tubs can be either set in concrete and a deck can be built around it, or the deck can be reinforced to accept the extra weight.
Please check that your deck is built to current building regulations. Decks above a certain height may need footings and extra strengthening, railings have to be constructed with specific measurements for support posts and railings, and steps and staircases have theri own sets of regulations.