For those wondering if they should purchase this piece of dlc (priced at 10 usd), here's some quick info on it.
 
Six factions total, paired off into three families.  You and your partner faction start off allied.  The families are located on three specific spots: two where the Mori are in the normal game, two around Tokugawa's turf, and two over by Date's province.   Each family has their own specialties as usual.  The Fujiwaras are better at civics, Minamoto at military, and Taira at naval and...something else that I can't immediately remember.
 
There are new mechanics.  Instead of religious influences, now you have family influences that are spread by this campaign's metsuke analog.  When your family influence reaches 50% in a province, they switch to your allegiance which is a fancy way of saying closer diplomatic ties.  Also, at that point, your metsuke analog can request said province to join your faction outright.
 
The map is the same, as well as province borders.  The "divided empire" mechanic from the vanilla game also makes a return.  Gain too much fame and the emperor will make all other factions become at war with you, including your partner family faction.

@Turambarsaid:

The "divided empire" mechanic from the vanilla game also makes a return. Gain too much fame and the emperor will make all other factions become at war with you, including your partner family faction.

This is weird to me. I would expect the DLC to be the 'official' way to allow players to play the game without worrying about realm divide. it seems they're quite confident of the feature despite its divisiveness... no pun intended.