Cube is when you create your own set, most cubes are done by using max 1 of each card and using powerful cards.
As you will be creating you own cube you can go for any theme, but do try to make it so that the colors are balanced and there is no cards that are always first picked or last picked.
It is also possible to make a cube in a way that you build your own set, take 4 of each common 2 of each uncommon and 1 of each rare and shuffle up boosters before play
No matter how your cube is created, it is commonly possible to manipulate the boosters in when you have this.
For example if you have a cube that is created with powerful cards having maximum 1 of each card you could make the boosters so that each booster has at least 1 of each color, 1 colorless and 1 multicolored by setting aside those cards before you mix up the rest of cards needed for your boosters. This does help to make boosters more even.
See the following link for inspiration and for testing out your own cube
http://www.cubetutor.com/
What would be the most interesting cube theme for you?