Just go into your data->world->maps->campaign->imperial_campaign folder and click on descr_start.txt. Right at the top is a list of unlocked, unlockable, and unplayable factions. Just Copy and paste all the faction names to the 'playable' section and delete them from the other sections.

This method also lets you play as the Pope, Aztecs, Timurids, and Mongols (the .cfg file only unlocks the unlockable factions) though I don't recommend trying since they aren't properly implemented.