I think it is high time that NWScript get some proper language features.
* direct compiler support for arrays or lists
* closures and higher order functions
* proper libraries instead of the current #include
As for builtin functions
* functions to modify ability scores directly like the dm_modify* functions in DebugMode
* functions to change hitpoints, Attack Bonuses, AC, SR, etc
* LevelUpHenchman to directly support a 3rd class. Currently it is impossible to get 30 levels of Arcane Archer with it.