Besides some that were already mentioned, I have these on my profile:
Are you sure, postcard is suitable for those themes? I mean- the space on the postcard. And must I, in my 60, really remember the last thing I learned in school?
My favorite questions are:
-What is your favorite fruit or breakfast?
-What’s the last song you’ve heard?
-What’s the last good book you’ve read?
And this one I ask to all new acquaintances when I can:
What two superpowers would you choose and why?
I would believe it’s possible to write to write about all those topics on a postcard, given that the handwriting is small enough. I know that some find it difficult to write more than 3-4 sentences on a postcard because their handwriting is so large.
Since the suggestions mentioned by @brittanyjewell are all mentioned by the Postcrossing Team in the Help section, I guess they’ve thought it through and only suggested things they know is possible to write about on a postcard. But as I already mentioned, the handwriting is a factor, and most topics can be challenging to write about if one struggles writing small enough.
Someone will command the font size for the whole world to write now? It would be funny if it weren’t so pathetic.
These are just suggestions to help people. You are free to write what you want.
Calm down. No one is commanding anything! In fact there are a lot of people struggling with what to write on the postcard. So Postcrossing provides them with suggestions. You are nevertheless completely free to write whatever you want. And how much you want. And in which size.
I transcribed what was included in the main PostCrossing website.
Ok… And sorry. But I think, if something is not very close to reality, it doesn’t helps. Even, if transcribed from PC website. Not everybody can insert a big topic in the size of postcard. Especially a person who already has difficulties expressing himself and telling something.
I have a sugestion. Not so much a subject… more like a component (for the future):
Would it be possible to have these suggestions appearing randomly when we draw adresses and are taking a look at the recipients profile? 
Much like the “hello!” in different languages that appear on top of the website. This could be something like "Don’t know what to write? What about… (insert subject/question)?". Or something like that. 
Not as an order, just like a random and friendly suggestion.
Just because it’s not close to your reality doesn’t mean it can’t be close to someone else’s reality.
nobody is telling you you have to write a long story on your card. this topic is just for people that want to but are out of inspiration.
@-Alter3ch0- i think that is a great idea. sometimes i draw an address that i have nothing in common with or my head is just empty and a random list would be great then.
Yes, this is a thoughtful idea. In the beginning when we started with the writing prompts, our fear was that everyone would be writing the same thing on their postcards, over and over… so we tried not to make them super visible, to not run that risk. 
After a couple of years though, I think we can say our fears were unfounded, so maybe it’s time to push them a bit more, so that more people are aware of them? Not sure about serving them with every profile, but somehow making them a bit more visible. We shall brainstorm ideas to make that happen!
OK, my two points. Maybe they also can help to somebody.
“your annoying neighbour” 
I could fill several pages. Brilliant prompt 
I love this idea! 
Prompt: How did you celebrate your last birthday? Or what would your perfect birthday look like?
What’s your favourite day of the year, and why?
2 truths and a lie about yourself. When I register the card, I’ll guess which is the lie.
A boring fact(s) about you.