I'm interested of making experience based maps to be played. can mexuiz map format add free variables to player characters to be handled as skill tree and experience? I'm also interested about autobalancing and player skill level database. Maybe some game balancing features to make game more interesting for newbs. Could I be part of development team with these ideas?

If you implement it, yes.

But, there are many worries about such a system. For example, privacy laws in some countries may render hosting a Nexuiz server that keeps a player database illegal.

As for additional fields, you can make mapinfo read in more fields (like skill level for the map), but how would you use that field then?

Wait, how it can be more illegal than hosting a forum?

Might be because it will have to retain IPs along with possibly encrypted data.

Might be because it will have to retain IPs along with possibly encrypted data.

I doubt that. Nexuiz client could be configured to use some security ID and besides IP based identification sucks with NAT connections. If server is being run in country which allows such identification hosting I'm quite sure there is no problem with other countries laws then.

Skill tree implementation to maps would need supporting map editor and I doubt there is any. Am I right? So it would also need editing to GtkRadiant. Am I right?

Simple. With a forum, you have to agree to the forum storing info about you when signing up.

In the game, you sign up nowhere. So if it tracks stats for anyone who joins together with either their IP or their nickname, it can be illegal.

For a legal way to track stats, store them on the client (in cvars that are written to config.cfg). But there's only limited space there.

What about frik file extensions? Or are they not applicable to client?

they are, and would work too, but I'd recommend using the db_ functions instead (which are based on FRIK_FILE and manage a hash table).

Now that I think this experience thingie, map modification for it doesn't sound good option. It has to be server side option like mutators in Unreal Tournament.