So, I've finally figured out what my missing ingredient was to adding new factions to the DB (it's always a dumb little mistake)
I would recommend doing it through the Assembly Kit with its automatically generated ID numbers. You need to modify or add your faction to the:
1. factions_table
2. faction_banners_table
3. faction_rebellion_units_junctions_table
4. faction_to_mercenary_set_junctions table
5. faction_uniform_colours_table
6. commander_unit_permissions_table
I would recommend copying and pasting the info related to units based on a faction with the same military group type (column in the factions_table). So, if you made a faction that had the roman military_group, just copy Rome's info for the commander_unit_permissions and faction_rebellion_units_junctions (Rome doesn't have mercenary sets, but the same logic applies, though I don't think it's as rigid as the others). This has gotten my new factions working, but you also want to match the faction up to a group in the
faction_to_faction_group_table, and it should be possible to create civil war factions for fully playable minors now by linking a new faction to the playable
faction in the faction_civil_wars_setup_table (you would also need to create the political parties.
Then add your faction into the start_pos as normal, and follow the rest of this tutorial to tweak your faction in other ways (icons, effect bundles etc.):
http://www.twcenter.net/forums/showt...ly-Kit-and-PFM
I will try to repeat the process in Attila, but I would imagine it will be very similar.
Hope this is spurs on some creative modders and overhauls to try new things.