I am a college professor, studying the psychology of people at work.
If I had a preference for topic, it would be related to working, whether it is a postcard of a factory or your company or something related to your job or a labor leader or picture of people working. If you are a student, you could send a postcard of your university.
I really enjoy art cards too, art from famous artists (especially Picasso who I respect because he was always changing his style) or artists related to your country. . .
Since I study people working, I would love it if you would describe your job and what you like or dislike about it. Or if you're a student, mention what type of career you want.
Buddhist/Buddha (my religion)
Cards in honor of protecting our planet against climate change
Cards related to peace or political protest
Cowboy and Western movie stars
Artwork from a nearby museum
Famous battlesites (let's end all war!)
People who inspire you (tell me why)
Religious sites (regardless of religion)
Cards related to your county if from USA (love county courthouses)
Politicians (describe who he or she is)
Anti-Trump (or heck, any pro-Trump if that's your thing)
Fruits and vegetables and other vegan foods
Small birds (especially the cardinal or chickadee)
Paintings, paintings, paintings!
Homemade art cards are fun too!
I prefer obscure things instead of popular things.
Here are some of my collections:
P.S. I enjoy tags and trades as well. I encourage you to check out the Tags Forum (especially if you are new and aren't getting enough postcards as you want). . . https://forum.postcrossing.com/forumdisplay.php?fid=68