Minus a few inputs such as flight spoiler, flap/slat setting, gear control, autobrake selection and thrust reverser deployment today's aircraft land in automation. Taxying to the desired gate is also a human function.
Recently we have created a system known as FANS "future air navigation system". FANS is sold as a system to text high altitude reporting commands to avoid mis-communication over forign land, this system could simply be modified to control an aircraft in flight via FMS commands to Flight Control Systems.
The simple imputs listed above would be easy to automate, the only limitations are FAA/JAR, etc. restrictions to pilotless commercial aircraft, the military operates "drones" as we speak, the only limitation we have to a commercial aircraft operating without a PIC is the liability and blame of loss of life as a result of a crash and loss of life.