You can change anything regarding the appearance/color/text/positioning etc etc although a program capable of editing .wnd files is needed - Im hoping GUIEditl will be released as one of the tools along with WorldBuilder - its tricky to get used to but it does all you need. Not sure on adding new shell options, although there appear to be none to actually add over and above whats there already.
New factions are possible but tricky to do and much editing of several files is needed to achieve this - early on, it could simply be done through the Faction.INI file although the core code for the BETA and therefore the final release will not read that file
