It really depends on how the game is set up.
WoW had a standard 16slot backback that Blizzard hardcoded to a ridonculous degree. It prevented them from upping it's size for many years, b/c they actually had to rewrite a ton of ancient code that was "held together by duct-tape" in order to make it happen.
Games are complex programs and esp after many years and patches, the original people might not be there any more and it is difficult for the current folks to understand everything.
Adding more faces / customization options may be sth. that SE really wants to do but it may very well be that the effort is just too much to actually make it happen w/o sacrificing other, more important things.
