I've been working on my "Sr. Design" for about 5+ years now, about 1 or 2 years to go and I can retire it (oops, I mean me).
First a couple of do not do:
1. Elevator
2. ASAR Car park
3. Washing machine
4. Anything "Green"
You may want to consider an assembly machine, maybe something simple like the assembly of a flash light or ink pen. Use a rotary table and two or three assembly stations. Maybe even an inspection station.
In my part of the country about 80% of the systems I see built are some form of this. I have been involved in everything from hand grenade triggers to car transmissions, engines and whole automobiles. The only real difference is size, speed and accuracy. If you plan to work out here it will be good experience and if done correctly a great enhancement to your resume.