So basically what I envision is a "training school" for your character. Maybe an old sensei/master (like Pai Mei) who talks you through many pieces of wisdom (battle tactics) collected from the myriad of posts over the years here on GB and on other forums. Then to illustrate, he either fights you, or puts you in fighting situations illustrating particular tactics. It can be set so you fight as a group or one-on-one. A challenging set of trials, "gladiator-style" (e.g. generic equipment, limited space), would be nice too, earning you experience. If successful, at "graduation" you would be presented with a powerful item or gain/stat upgrade/special ability. Perhaps a mark too that you belong to Shaolin, the White Lotus, or whatever
There are several mini-championship challenges already in the game (Druid Grove, Copper Coronet, Underdark, the Ritual mod), but (with perhaps the exception of the Ritual) they didn't feel very challenging or training-style.
The [weak/mediocre protagonist => intense training => powerful protagonist who eventually wins] paradigm is present in a lot of martial arts/fighting movies (including Star Wars), and I feel a similar "academy"-type training facility with a powerful & strict master teaching you the secrets of combat, would be a nice addition to BG2.
Of course, this is a very rough idea, and only an idea after all, but I would greatly appreciate your comments.
-kaitsuburi