The first thing you should do when designing anything is to brainstorm all of the features you want and in order of importance for example it should look something like this:
- made out of a bath tub
- Large opening door
- flat top of plant pot top
- wall hanging
- Bath tub feet as handles
As you may have noticed several of the features are dependant on the material. So depending on the material it can be useful and easier to design your piece of furniture around the material, however this can limit the possibilities of the design.
Once you have brainstormed all of the features you would like to be in the final design you can just go crazy and begin to draw (or model) your ideas.
Once you are happy that you have explored every possibility you can decide which one to make based on looks and the amount of features that it has.