It might be helpful in future, as games become more network-based and platform boundaries become more blurred, to have XML languages defined for some common game design elements.

rpgML : Allow the definition of rules and data tables, objects, characters and character classes, scenarios, locations, universes analogous to paper-based RPGs to be concretely defined and extended for the use under different game engines. For instance the entire AD&D rule system could be encoded and rpgML document-objects generated to play games under those rules...

avatarML : (although I hate the term Avatar for virtual character representation, apart from in Ultima) define the appearance of a character, so that the client (for instance in a 3d shooter or MMRPG) can generate their appearance to the highest available level of accuracy with the minimum amount of network traffic, and so that characters can be taken from game to game (although an extension would be required to store the rest of the character's attributes -- optional, as many games will just want to LOOK the same, and the games will have vastly different sets of statistics depending on the style of play).

More to come... possibly... (sports games in particular being a mine of statistics...)