My first walkthrough had a balanced party. So lately I've been experimenting with a very unbalanced one for fun. I started with all ranger builds, but gave them all summoning instead of pyro. I mixed up the races to get some of the benefits, and tweaked the stats so specialize each a little. For instance, one focuses on thieving/sneaking, another has improved melee capes, etc.
It seems to work pretty well. I'm able to jump-in to a level 2 fight right away and win without too much trouble (everyone summoning minions). I'm not sure how it'll play out later in the game.
I'm interested in other concepts that folks are trying. I started using unbalanced builds in Pillars of Eternity and it's a lot of fun (though some of them are very OP).
