These proposals are ready for planning: each one has a recommended platform, prerequisites, measurable success criteria, phased parts lists, safety gates, and links to established kits and open-source code.
Start with the ground vehicle, or take on the microdrone after basic feedback control and telemetry feel familiar.
Build a small remote-controlled car, then add telemetry and speed feedback as a robotics foundation.
Build a roughly 9-inch (230-250 mm) motor-to-motor quadcopter with 5-inch propellers, PX4 flight control, telemetry, and a logged autonomous mission.