Thought I'd start this thread, because BI appeared.
Here's some basic info, posted by gemuse over at TWC.

Quote Originally Posted by gemuse

Just got BI and I'm currently looking through the Data folder for new stuff. Here is what I've found so far:

- There is a new way to set up faction relationships in descr_strat, seems a bit more advanced. Now you can set
the relationships on a scale for 0 to 600, where 0 allied and 600 is war.

- To my untrained eye it looks like the cultures have been "un-hardcoded", there is a file descr_cultures (was it there before, can't remember seeing it...) which describes how each culture loks like on the campaign map. Also in the file descr_sm_factions, you can set which culture a faction belongs to.

- Victory conditions seem fully modable, at least for BI, not so sure about the Imperial Campaign

- It's possible to set brigand and pirate spawn values from descr_strat

- There is an attribute in descr_sm_factions, prefers_naval_invasions, which could be interesting ;)

- There is a new file in the bi campaign folder, descr_harvest.txt. Here you can specify bad harvests, by year, for regions, factions or regions if owned by a faction.

- The bi descr_strat contains commands to disable the reforms and glad. uprising and enable night battles (would be interesting to try these on 1.2), and something which activates rebel chars.

- The imperial campaign descr_strat entries for the roman families says thet the senate is a superfaction of these factions. Could this be the key to splitting up the romans? The bi descr_strat doesn't contain this attribute at all.
This info is helpful. Should give us some ideas before we get our greedy hands on BI.