I wouldn't think that arranging a duel was that hard to begin with. As nice of an idea as it may seem, it suffers from some serious flaws which will prevent it from becoming popular no matter what tweaks are made. They are the following:

• Lack of adequate servers. For this idea to work, a server would have to be capable of handling a lot of clients playing at the same time, which is hard enough to find as it is. And most serious duelers would prefer not to duel with the extra lag caused by other players being in the same server. If fast dedicated servers were readily available, they would be more useful if only two players could play at a time with the hosted level chosen by the participants.
• Excessively complicated levels and hosting procedures. Triggers are highly recommended to make sure players can't go into the wrong arena and cause problems. And the trigger system that would be needed (in addition to all the arenas that must be made) can easily make a level become very complicated. But whether or not triggers are used, someone will have to be around to make sure nothing goes wrong. And something going wrong is very likely to happen. This flaw isn't as much of a problem compared to the others if the right precautions are taken, but it still can be a big issue.
• No practical way of handling CTF. A quick glance at the Expanded Round Stats on JDC will tell you most duels are played in CTF. There isn't a simple way of making multiple sets of CTF bases work in a single level, and considering the other flaws of this idea, it's not worth the effort.

By the way, servers only allow 15 seperate connections (there can only be more than 16 players in a server if at least one of the connections or the server uses splitscreeners). For this reason, it would be practical to make only 8 arenas.