Can users propose a full tree of rankings?

Not directly. Designing a new tree can be very complicated in some cases, even for a person who knows the system well. In order to make the task as easy as possible to the users, it works like this:

All a user has to do is think of an item, and then of a single ranking for this item. He can then repeat this step if he wants to add the item to some other ranking(s) as well. Now, the difficult job of how to arrange the tree is to be done by an OpiWiki moderator.

For example, a user wants to add Muhammad Ali to boxers and heavyweight boxers rankings (assuming these ranks don't exist). He just creates one ranking at a time, not worrying about the scheme.

If a user is experienced enough to have an idea of how to arrange the tree, he can propose this in the info for a moderator field (create new ranking menu).

If you believe some scheme could be designed better, you can propose a change, here, on Meta.

Only possible problem would be if you cannot rearrange it later since a badly thought of structure may fragment information and make it impossible to find. That's something I've sensed when navigating the author/writer/novelist and their subtree parts of the realm. A full classification tree (as a tree) is most probably impossible, libraries kind of realised that, and Wikipedia's category system is more of a graph than a tree, plenty of links and cross-references. Be prepared. :-)

Yeah... As a melomaniac I am scared to see too many genres of music being misused as rankings, like speed metal, black metal, death metal, etc.. I am not against adding these rankings, but I am afraid to see people, who don't quite know what they are talking about, messing around with them. Plus, some rankings, and not just in the music category, can be up to interpretation. I believe you will need a lot of moderators who can form "councils" and vote for the existence of certain types of rankings, or simply create "meta-opiwiki defining polls" where every single user is allowed to vote for or against stuff like that.

I thought this question could fit in here...

Is there a way to see ALL rankings in the form of a tree, on one page? Because opi.wiki/rankings only shows them separately as "buttons".

When I'm editing articles, I can see the item's current rankings in a much more efficient way...

Ex:

• Artist
  
• ...Musician
  
• ........ Guitarist
  
• ............. Rock Guitarist

1- Add any ranking that seem to be missing from the current list.

2- Add existing rankings I might not have been aware of (or thought of) while creating an item.

Because, at the moment, whenever I add rankings after creating an item, I often feel like I'm forgetting about a ranking or two. It actually happened to me on some occasions already.

If I could avoid coming back on the items I've created to add the missing rankings, it would save me a lot of time, plus it would give OpiWiki better, more solid looking pages overall if the page had all the required rankings right upon its very creation. Rankings seem to be very important for this website because if an item has no rankings, it's almost bound to be lost in the database until someone actually searches for the said item directly.