I would like to request that a topic about this be placed on the FAQ. I've seen multiple threads but it still doesn't clear the ambiguous text on the kensai's page and I haven't seen any confirmation from the FAQ to know.
My question is, if a Weapon Master (Fighter) can pick any weapon, regardless of proficiency or not, and a Bladebound Magus can pick any type of blackblade (So long as it meets the weapon requirements) it can be Simple, Martial, or Exotic, regardless of proficiency, why don't we get any clarification on what limitations the Kensai has with weapons he can and cannot pick? Considering also that Weapon Master can pick anything, and the reasonable assumption that the two archetypes are very similar (With the exception that Kensai must pick melee weapons, which even itself is very vague) that a Kensai could pick a simple weapon as his chosen weapon.
The only thing we have to go on is vague text that doesn't explicitly say for or against this case, and find correlation between two places we shouldn't have to. Even if a kensai couldn't pick a simple weapon, it'd provide less argument and issue if it was simply worded "When becoming a Kensai, pick a single martial or exotic melee weapon. In addition to all simple weapons, you gain proficiency with this weapon, and it becomes your 'Chosen Weapon' for the purpose of your abilities that affect it."