I would suggest the way WoW is going: You can implement things like a good Auction House experience (also with detailed info about items that were sold / not sold and some graphs). Or things such as damage and threat meters.
But there are many different styles how to play a game like this. I for example love to be able to cusomize my UI and datamine pretty much anything by using addons. I think you should give people the possibility to do so, as long as it is generally open to anyone and no one would get an advantage over others.