When you are going to build a robot, regardless of how small or large, simple or complex you think it might be, you always start with a seed of an idea. How you grow this seed before you build will be your design.
You may take it as far as creating a full model in a CAD package, and prototyping with a 3D printer, or you may simply draw a couple of freehand sketches. Whatever you choose - do not skip the design phase.
This is a chance for a team to get all their good ideas down, then combine or reject them, flesh them out, tune them, and try and constrain them to get a good direction. You must ask questions - what are the advantages of this, what are the disadvantages of that? What potential pitfalls or complicated parts are their to this? What are the weaknesses and strengths of the chosen build materials and techniques? Can you finish it in the allotted time? How long do you expect the full build to take? How long might it take to fully flesh out and understand the build?
You have to think about if the materials you need are available, and what they will cost, what tools will you need to work them? What safety equipment will you need?
Only when you have satisfied these questions are you really ready to head to a workshop and start building - even for the most basic Lego robot, a little design and forethought may save you a few hours of frustration. It also helps a team work well together if a design and basic direction are created beforehand, for them to stick to. Once agreed - a team can get on with it. Of course - be prepared for the fact that even the best design may not work, and you may need to return to the drawing board. By at least having an initial design - you can see what needs to be changed, and only alter that.