I love ALL the postcards that show up in my mailbox, but if you want a few ideas...
I enjoy postcards of maps, flags, coats of arms, traditional clothing, and landmarks such as cathedrals, bridges, temples, & natural wonders. I'm interested in German language and culture, the former east bloc, communist poster art, royalty, portraits, celebrities who aren't famous in the US, beards, vintage fashion, mythology, Olympic games, metro maps, unique regional traditions/festivals/customs/clothing/folklore etc, religious iconography, anatomy, skeleton keys, Japanese woodblock printing, linguistics, urban planning, office supplies and you.
Folks from the US: I'd love a map card or one of your state flag.
I am totally interested in direct swaps.
Looking forward to seeing you in the mail.