I have been playing with prefixes for a long time now and as a result I can say that just adding prefixes for the sake of adding them is not a good idea. There have been cases prefixes worked out well, but other times they were a pain. If they do not serve any real practical use, I'm not going to add them.
The ones for the hospital and the battle tests section by default I'm not going to consider. It's not like we get a hundred transplants and CM each week. There is no point in starting using prefixes in such a section, especially as the only ones who actually need to work there are the med nins and that are only a handful of members. They really don't need to have 19 threads saying 'treatment' and 1 saying 'CM'. Similar thing in the Battle Tests section. The only people who actually work there are the ones who are administering the tests and as you're already obliged to specify exactly which test you are doing in your thread title, adding prefixes becomes redundant. There is no practical point in knowing that one test is for a bio and another for a sensei as the test administrators already know that.
So then you only have the prefixes left for the Arena. A 'tag' prefix is not going to be made as tag battles are not the limit, you can also fight with more people. Also the SY tag rules are for the NW, not the Arena and I have the feeling that many people seem to be mixing the Arena and the NW. Also a 'tag' prefix conflicts with the 'open' and 'closed' prefixes as a tag battle has to be one of them either way. A 'tourny' tag I don't find useful either as I can't even remember when the last time was we did a tournament there. Those are these days mostly held in the Battle Tests section. When we have a tournament going, it might be useful to add it at that point, but it's not like we have tournaments continuously.
So the only two tags that are still left are the 'open' and 'closed' ones. People these days seem to be using them all the time, but realistically I really do not understand as why and I got the feeling that is again one of those things that originated from the NW and is alien to the Arena. I mean you can't just involve yourself into people's fights, when both parties have agreed, you can't intervene. So an 'open' battle becomes a 'closed' battle after one post. You just need to find a person to fight and that's not going to take long, so after one or two posts your prefix isn't even correct anymore. And closed battles are in fact standard battles. Normally you already have someone to fight and when you have someone, other people can't just intervene. In reality these 'open' and 'closed' tags people use are completely contradictory to their intended purposes as what are the threads then that do not use them?
You also have got then the issue of making prefixes mandatory or not. Not making them mandatory, as I just said, is confusing as what are the threads then without the prefixes? And making them mandatory makes all the threads with the open tag a lie after one post.
We've never had any kind of issues with battles in the Arena, most of the time you can also deduce very easily from the thread title whether someone is searching for someone to fight or whether he already has someone, so I don't really see any reason to change anything. Honestly said that people should stop using those 'open' and 'closed' tags seems much more beneficial. I mean a thread has 15 posts and it has the prefix 'open'...well obviously it's not open anymore. It only serves a purpose till an opponent is found and then it becomes a 'closed' battle. So using 'open' and 'closed' is more confusing to me then not using them.