I think something that would be very useful, is the ability to redefine the abilities of any XML, while leaving anything not specifically redefined exactly the same. This would be hugely useful for modding, and choosing when you want to add abilities/stats but don't want to include the whole file, like when you want to edit DLC items without providing the item to those without the DLC.
This would be done by adding _REDEFINE in capital letters to the end of the internal name. If no InternalName exists without _REDEFINE in it, the code would have no effect. But if it does - say Internal Name is "ChampionInternalNameHere" then the redefine function referring to this Champion would be "ChampionInternalNameHere_REDEFINE" and explicitly changes ONLY those tags/abilities that are included in the _REDEFINE. But if "ChampionInternalNameHere" does not exist in the core or mods folder, then "ChampionInternalNameHere_REDEFINE" would simply be ignored with no error. This is so you can redefine files that may or may not exist in a given installation/associated mod folder without having to create a new version for every combination of core files, DLC and mods that players use.
So if you have a _REDEFINE function with changing the UnitStat_Attack_Pierce of L1 or any other level (in the case of monsters who are not champions), then that would overwrite the existing Physical Attack of that unit. It would never ADD to an existing ability.
Also, to make such an ability truly comprehensive, you would have an ability within a _REDEFINE InternalName section of code to "Delete All Existing Traits", "Delete All Existing Unitstat Level Definitions", "Delete All Existing Equipment", "Delete All Existing Treasure" or "Delete all Existing GameModifiers" and any other ability which can have multiple items, which you would use to delete all the existing abilities in each of these categories, and would be followed by the Traits, Unitstat Level Definitions, Soulbound and Transferable Equipment, Treasure and GameModifiers that actually DO apply to this InternalName.
This would affect any core files, DLC files or mod files loaded before. So you would be able to redefine Quests and RandomEvents too. Each InternalName definition would be affected by any InternalName that is exactly the same except it ends with "_REDEFINE".
So the two functions of _REDEFINE:
1. redefine any InternalName without touching or nullifying anything that has not been specified in the _REDEFINE code; and
2. have an option to delete all the existing characteristics in a category. Such as traits, Unitstat level definitions, equipment, treasure, or game modifiers in the case of spells, techs, and redefining traits (eg. deleting all existing gamemodifiers) - so you could in the _REDEFINE, delete the existing elements in one or all of these categories and then add the ones that will be inserted from a blank slate.