Which of these sounds best for an aviation-inspired Extended Project project?

I think you should be careful not to pick too broad of a subject thinking that that would make it a bigger project - if you pick something more specific i.e. the hydraulics versus fly by wire example you could go into a lot more detail here.

You could look at the history of flight controls, assess different aircraft systems and the pros and cons of each, and look at case examples of crashes which have resulted from failures in each system.

If a full fly by wire system fails, how does the pilot control it? What redundancies do hydraulic powered systems have.

You could branch off and do a section on the airbus looking at its flight controls, and the different flight modes etc. Look at the air france crash as an example maybe.

You could discuss how they have benefited pilots etc.

How the airline industry has changed our lives could be interesting although it has probably been done before - but you could look at the benefits (industry, news, travel etc which it has helped) as well as the cons (environmental) and discuss how the aviation industry may change in the future (look at the expansion of the middle east market, discuss the problems facing places like the UK with no more room to expand despite growing requirements)

You could look at environmental factors - and look at how airplane manufacturers are addressing this. New engine designs, biofuel etc

You could do an interesting project on automated flight versus human pilots - discussing the systems required for fully automatic flight and the likelihood of unmanned drones in commercial aviation (what is preventing this, how it would be good for safety etc) and also do a section in this on human factors - when human pilots are good and also how they are often attributed to the mistakes.

A research project needs to be more than just copying out the data of what is already out there - you need to think of a question your project is hoping to answer. For example, looking at the environmental impact of aviation, you would hopefully want to answer a question at the end of 'how can we improve the environmental impact' and include suggestions for future improvement.

If you do a project on the future of aviation you could answer a question like "how is the aviation industry likely to change and how can airlines keep up with this and ensure they continue to develop and be successful'

Try and find an area you are really interested in and don't pick too broad a subject which will be too hard to fully research.

I think any well presented, well researched project will impress employers so don't worry about that.

The Fly by Wire vs Hydraulic/Mechanical Systems can be expanded to an issue that is under heavy debate:

The computers that run the Fly by Wire systems are getting very sophisticated. The issue is: The computers know how to fly better than a human pilot. (?) Should more authority be given to the computers to prevent Pilot Error?

Should an airliner be "stall proof"? The pilot operates the controls in a way that would stall the aircraft at low altitude. Should the computers refuse to do it?

With GPS the autopilot system can land an airliner smoother than a human pilot. Who should be in control for a landing? The pilot or the computer?

The GPS-Autopilot System above does NOT need any of the existing Instrument Landing System. The aircraft can be landed safely in 0 ceiling/ 0 visibility. Should the ILS and Minimums be abandoned for GPS-Autopilot equipped airliners.

Is a human pilot even needed? Why and under what conditions?

Should the pilot be able to shut automated systems off? If "yes," under what conditions? If "No," who is responsible for if the system fails?

Should the pilot even be in the aircraft? Should airliners be flown as drones with the pilot on the ground? During cruse phase flight, the autopilot does the flying, the human takes over only for the critical phases of flight ====> One pilot can fly several airliners (?).

Etc.

Extended Project Examples

It sounds like this is a research and writing project, not an experimental one. I encourage you to study flight safety, and what changes improved flight safety. Read the reference, especially around page 250. Perhaps look at deployment of various kinds of air traffic control systems and radar. I think a potential employer would be very impressed by your interest in safety.