From the update:
In a continuing effort to give you what you want, we've added a new Crystal Chest to the mix: Prismatic.
Prismatic will allow you to pick the models You Want. There are a few limits however, its only going to be available to Thieves, Dragoons, Clerics, and Prismatic Knights. We're also going to put a soft limit of 1 Prismatic Crystal Chest per backer, as in we'd like you to only pick one and then fill the rest of your pledge with the standard chests, but we won't stop you from picking more then one Prismatic. Currently you'll be able to select 4 Models, at $25k you'll be able to select 5 Models, at $35k you'll be able to select 6 Models.