Before doing anything copy the original file.

To unlock factions in campaign, find descr_strat (mostly in C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\data\world\maps\campaign\imperial_campaign).
Open this file and move factions that you want to play from nonplayable and unlockable to
playable.
However sometimes this can cause problems when you try to modify the file twice.

Navigate to maps folder and enter battle folder. You will find several folders containing historical battles files. And same like before find descr_battle file and modify it the same fashion like campaign files.

There is a lot of things that you can do only by modifying existing files. The most interesting thing, is that you can create custom battles and modify campaign starting stats, like state ownership etc.