Great design and great idea. I don't know whether many people still look at this, but I discovered it today and if it was advertized and used more.

Some suggestions for Stackathlons:

 - Quantitive Finance and Economics. This one has suprisingly few, while the disciplines are quite closely related. I tried to add another one, but then it gives probably zero users. In fact, this query gives zero, so it's [tag:status-declined]

 - Mathematics, MathOverflow, Mathematics Educators (Math Masters) [tag:status-completed]

 - The Three Sciences: Physics, Chemistry, Biology (Science Masters) [tag:status-completed]

 - Astronomy and Space Exploration. (Space Fanatics) 

 - Ask Ubuntu, Ask Different, Unix & Linux, Super User (Computer Experts)

 - Stack Overflow, Server Fault, Super User, Meta Stack Exchange (Original Tetralogy). Did you know that Meta SE is older than Super User? Probably the largest tetrathlon. 

 - Information Security and Cryptography. Cryptography is of coure highly relevant to Information Security, since our data can be sent securily because of Cryptography. 

 - Biblical Hermeneutics, Christianity.

 - Beer, Homebrewing. This one may only give one user. 

 - Science Fiction & Fantasy, Movies & TV. This one will probably give a lot of users, but I don't know what to add to it. Maybe Anima & Manga, but I think it then will collapse to zero.

 - Travel, Expatriates. Living abroad short and long respectively. Both sites need experts on travel documents, so they are related. Travel.SE also advertizes Expatriates.SE.