It's never too early to explore design ideas.
After all, it's called "design thinking" for a reason — critical thinking doesn't happen only inside your head; it also happens on paper, ink, pixels, or whatever medium you prefer.
If someone tells you it's premature to begin designing before a problem is defined clearly, they don't fully understand the problem-solving process. Problem solving isn't a linear process. It's a messy circle out of which eventually shoots a clear path forward.
One of the most effective methods I've found for solving a problem is to dip my toes lightly into the solution once I have a basic, rudimentary understanding of what I *think* the problem is. As I start solving the problem, I refine the definition of the problem further. Solve the problem a little more, refine the problem a little more, repeat the loop.
Problem-solving is winding and unclear at times. And that's a wondrous thing for creativity and innovation.