Has somebody some good ideas for a new Raspberry Pi Project?
Or something whats really useful?
(Something without buying stuff just with the rpi)

I use mine as a local file server

Really not that much that you can do with just the pi.
The usual is a crude media server or web server, or any other server.
There's literally nothing else worth doing with it if you have a regular computer.

It's really not that expensive, and quite rewarding, to get some electronics and start an IoT project. You'll learn a lot in the process.
Though you could kill your cat if a hacker hacks into your IoT automatic meow-activated cat feeder, so be careful with that.

Install pihole, config router to RPI as new DNS, then use internet as normal with no ads.

implement a port knocking tool which connects to your wifi router and enables WLAN as soon as the correkt portknocking sequence has sent to the Pi. And use a TOTP mechanism so that guessing the ports will be impossible and you have to call each time a new sequence..

well, maybe replace the WLAN part with whatever you want to do (and is a hidden service, unless you want to use it).

@agentQ Already using it but I want to have more things running on him & on the other is my file server

@karwler too & a other for pihole

@AndSoWeCode Already using it as a file server & a other one for pihole

@2erXre5 Nice idea maybe I will find something useful

Setup an open wifi network for people around the neighbourhood.