So I would take a step back a bit. What made you pick a shopping cart as you Capstone? Do you feel this is a challenging and meaty project for you? If so what is the root of the problem here with shopping carts?
I personally don’t find problems with the cart itself, but rather the entire shopping experience. The traffic jams created by the carts, the check out experience, like you mention the multiple times of items going in and out of the carts, and this may sound dumb, but when it rains, snows, or is just plain cold outside the carts sit outside and bring the weather in with them. Not pleasant.
Can a new cart address the issues I mentioned above, maybe, but I would think of it as designing the experience and the cart is only part of that. By thinking this way it turns a project that I would look at as mediocre project, to one that has a lot of robust thinking and shows you think further than the artifact.
Post your process so we can give you feedback.
J