I would use the wagon wheel as a gate or to grow a climbing plant on.
Or you can be extra creative and make a large scale of the design and use timber for each section this will hold the soil in, in each section have walk- ways and the centre representing the cog have a feature plant. This will most likely take a lot of room.
Mark out the outline of the wheel. Are remove all the weeds and grass. Place the wheel flat over the cleaned out soil and plants the plants inbetween the gaps.
Cute! I've see it where someone laid the wagon wheel down and in each space between the spokes, they planted a different herb. Very cute!
Here are several ideas for wheel planters https://www.pinterest.com/pin/759771399607414079/
this post has a lovely wagon wheel plantes with succulents http://www.homedesigninspired.com/20-truly-cool-diy-garden-bed-and-planter-ideas/