One college-level science fair project idea is to try and prove or disprove that chemical pesticides negatively affect plant yields. Another idea is to theorize and build a model for a contraption that can safely remove plastic from large bodies of water.
One idea for a college-level science project is to create a biodegradable or reusable food or beverage container for snack food or sodas. Along the same lines, a good idea is researching and theorizing ways that land fills can be cleaned up or reduced and how that can impact the environment. Any project that might address reusable fuels or conservation of various habitats is likely to succeed. Alternatively, consider a discussion about robotics for household purposes. A science project that explores a unique use for robotics can definitely garner attention.
College-level science fairs are typically conducted over the length of an entire semester, so students have plenty of time to plan, experiment and execute a well thought-out idea. The idea can be as complex or as simple as the student likes, as long as the presentation and outcome are original and professionally executed. If the project is impressive enough, it can generate opportunities for employment or graduate schools.