Read the forums. You'll see a lot of repetition: aquarium controllers and door openers for chicken coops are popular. Garduino projects abound. Then too, there's the occasional project that's totally new (at least to me). For example, someone wanted to replace the controller to his hydraulic sheet metal press a few weeks back. A list of projects you found here ought to provide you inspiration I would think.
There's also hackaday, instructables (for concepts only - ignore the actual instructions!), Pi forums - some of those projects could just as well use an Arduino.
But really, it all depends on you and your skills and needs. Look around your living space - is there something you would like to automate? Do your hobbies suggest anything?
Edit: Is there some new hardware you'd like to master controlling? Servos, GPS, LoRA, stepper motor, solenoid, DC motor, flux capacitor? Plenty of mini projects in there.
We can suggest ludicrous projects for you all day long, but you need to give us a clue.