Sometimes I’m just registering domains because I think that specific domain name could be worth something. However, I don’t always have an idea for them.
A good advise is to check the domain with different extensions. Let’s say you registered my-domain.de. Now check out my-domain.com, my-domain.net, my-domain.org, etc. and see what they did with that specific domain.
That might help you with building an idea for your domain!