Custom special abilities rules
I would think that would really be up to your own experience that you want to have when playing the game.  Also, doing some play testing would be beneficial in getting a good understanding of whether or not it is over/underpowered.  I know it is a long game, but I would think after one game you would know more of what needs to be tweaked to determine the power of that ability.  And you could always offset that ability by providing lower stats, possessions, and etc.