Let's give this a shot
1. What edition do you play?
1. Currently 4th Edition.
2. Do you add any rules? (a rule that doesn't exist in RAW) Why?
2. Yes, I will. I find the additional rules by Heroes of Rokugan (HoR) very useful (I like their favor system, I found it very thematic and a useful soft control method), so I will adapt it in my next (also my first) campaign.
3. Do you hack any rules? (changing an existing rule) Why?
3. I will. First thing come to my mind is the Honor and Glory system from HoR which I find it streamlined.
Now, I will hack and slash the setting, creating/ eliminating new Great Clans and Minor Clans and re-skinning Schools as needed for my setting.