Ideas for MineCraft building projects?

I find myself getting repetitive lately, what with all the forts and all the underground tunnels. I need some new ideas.
Note: I'm NOT looking for things like statues and 'art work', I'm talking real things that could potentially have actual gameplay use (for example, I use my underground tunnels for traveling to other buildings without having to open doors).

Mine cart systems automated miners using tnt err I havnt played in ages a system that turns lights on during the night using daylight sensors there's a few.

I built a huge wooden fortress off of the ground, being held by the 2 walls it was between (it was in a canyon). At the top of the canyon I had a huge mansion that led down with watch out floors and a large ramp to the canyon floor.
I had a ton of fun shooting mobs and building it with my friends

Build a golem forcefield. Make a ton of cages around your house and put golems in them. When monsters get too close, the golems attack

I once made a town. Which had different places like a race track, a mine cart system running through it, a zoo etc. Took ages and I could add to it when I wanted something new and it was fun to walk around! It also had secret hidden underground passages which made it fun to explore