Don't waste your time on such a silly project!
Just off the top of my head:
- You can't mod the changed blesses back
- You can't mod changed spell or weapon effects back, as those are hardcoded and not accessable for modding; you can only choose between effects
- You can't change the castle building system, as this would require a GUI change (there's no castle choice in pretender creation)
- You can't remove the eras and bring themes back, unless you delete all nations and load every nation which is played in a single game via mod file
And I think it's good that you can't do that!
Why that, and why do I think it would be silly project anyway?
At first, lots of work has gone into Dom3 to make those changes and new features work well and balanced. Throwing that work away doesn't make sense to me. Furthermore, you won't get to see much of
how it plays with the improved UI and AI - as e.g. the AI does not know about castle choice at pretender creation any more. I'm not exactly sure what it's criteria choosing the provinces for castle building during the game are now, but for sure it tries to protect it's borders with castles, and it may go for ressourece-heavy or 'special' provinces as well, I'm not sure about that.
Oh, and o.c. you can't mod the AI not to build castles at all!
You can of course mod some of those inbalanced units and items back in, or create some new one. But I really don't see any worth in that - do you? Is clam hoarding really that great?