Hi there Panagiotis!
It is a good question, but there are so many correct answers to it ...
I would say that you can write about whatever you feel comfortable with and you believe that contributes to the community's content.
It is important that your content is authentic, with a personal approach by you (the author).
It could be a photo, it can be thoughts, it can be a theme about travel, about life, about science..
If you have not already, I suggest that you read this great post by @iliasdiamantis here where he answers to your questions in details and with great examples!
p.s. This phrase by @rouketas said it all ..

Just blog your things that feed your passion !!!