Good question. The pen needs to have the following features, at the least.

1) At high altitude, your fingers may change shape/texture compared to when at ground. For example, mine get a bit crusty. Don't know why. The pen should be able to extend a comfortable hold under both condition

2) In the event you sweat when answering any of the questions ("Are you bringing in agricultural products" or "$$$ more than the limit" etc.) due to reasons only you would know, it's important your finger does not slip due to the excess sweat - a good grip is mandatory

3) Very often, you will not have a table to rest your landing card on, because you forgot to fill it in flight, and made a mad dash to the customs counter to get ahead in line and now have to stand resting the landing card on your leg. So it is important the pen only requires the lightest pressure to start writing

4) Again, related to 3), it needs to be able to write with the nib easily upto 90 degrees perpendicular to your body

5) It needs to have a GPS along with a companion app to track. Because there will be someone who asks you to borrow the pen, and forget to return

6) It needs to have a magic ink eraser at the rear end because you will make mistakes and want to re-do, but the FA will never have an extra landing card for you and you can't fill these up with pencils. Of course, this also may happen if you keep changing your mind on what to declare and what not to (which I don't recommend, of course)

7) It needs to have a bar code scanner. Why? Because when you first get your boarding pass, you will scan the code of your boarding pass and when you are filling in the card, it will automatically display your flight number, boarding port and destination port, and date, because you will never remember them when filling up the card, and you just trashed your boarding pass stub anyway, didn't you?

8) Which brings me to - it needs to have a small one line display to show the details of point 7

9) Smudge free, of course, preferably with waterproof ink

10) Finally, it needs to have black lacquer with platinum rims. What sensible person would use a pen without these?