Any project that is related to computer vision and has difficulty appropriate for a 4th year course is acceptable. All projects have to involve an interesting theoretical idea, a challenging implementation and be exciting and fun.
Many students choose to program the robot to play an interactive game with a human. These typically involve hand eye calibration and require challenging heuristics to detect objects.
Another exciting class of projects involve human interaction with a computer vision system. The system tracks a human body part or an object held by a human with one or two cameras and responds in an interesting way.
Another class of projects involve the integration of an available computer vision algorithm in a project. The challenge here is to understand the algorithm well enough to explain it in your short presentation and be able to modify it and integrate it in your project.
Finally, one can implement some simple vision algorithm and explore the performance of a few variants. For instance, explore the performance of variants of the Canny edge detection algorithm, or a edge grouping algorithm that is based on EM clustering.