Could set it up that when you see someone in a server then it sends something off to the card server going "HEY I SAW ____"
If you didn't have them, it adds their card to your album
The less people that have a card, the lower its rarity is (based on a % of people who have card albums).
% of users | Rarity |
<1% | Legendary |
1-9% | Super-Rare |
10-25% | Rare |
26-40% | Uncommon |
41-99% | Common |
100% | Whore |
Then you can of course trade cards, and set a quote to appear on your own card, set what avatar parts should show in the image on your card, and maybe a user rating system. If you used rasterized images of the cards you could even post them on the forums!
This is of course a lot of work for something utterly useless (but cool, and that's what matters, right?)
And because I'm bored, a mockup of what said rasterized card could look like:

Clockturn
Rarity: Uncommon
Rating: 3.75
Rarity icons in order here, too