Is it illegal just to have a onion site running in the first place? That would be a cool thing to try but I am scared to dabble in that stuff.
It's not illegal no, but why doing so?
The only use of Tor and the darknet in general is anonymity. If you don't sell anything illegal, why would you need such privacy? Moreover, the vast majority of people don't use Tor, so creating a legal commercial onion website... Well I don't see the point!
There are some pretty ridiculous laws enforced around the world. One could easily set up legal business to help others do illegal things from another jurisdiction.