I feel like a new design should be coming with the next model considering if not, this will be 5 models that have used that same design. 2010, 2011, 2012, 2013, 2014.
I don't think how they could improve the 'design', it's perfect for me. Perhaps a different color (dark grey like the iPhone 5S would suit me) and a hi-res display (retina?) but that's not really changing the design.
I bought one 3 weeks ago and it's a wonderful machine.
What would you like to see different then ?