Is there a way for a weapon to grant proficiency in itself, like the training enhancement, but specifically whatever it is right now? I'm asking because I'm looking for a shapeshifter with a shapeshifting weapon.
Alternatively, is there a way to be proficient in all weapons?
Take a level in fighter. and feats for whatever exotics you want. Which should not be neccessry.
If you're willing to stick to blades you could go Tengu. They're proficient with all "sword-like" weapons.
As Drahliana Moonrunner suggested, any class with profiency with Simple and Martial weapons is going to cover nearly any fighting style that you want. Without a lot of Feat investment, there aren't many Exotic Weapons that are better than the Martial ones.
If you really want to be able to use the Exotics sometimes you can try this.
Get a weapon with at least a +1 Enhancement bonus, then add the Transformative or Greater Transformative Special Ability. (I think Greater Transformative is well worth the extra 5,000 gp). Now dip a level of Occultist (which does have proficiency with all Martial Weapons), and choose the Transmutation Implement. When you change your weapon to an Exotic form use Legacy Weapon to add Training (Exotic Weapon Proficiency (-)).
A gnome can be proficient with any weapon that they make themselves, with the right alt racial trait. If such a gnome makes the transformative weapon they should be proficient in it.
There is a way to apply them all to monk weapons.
If you take Ascetic Style and apply it to unarmed fists which would normally be reduendant and then at least 5 levels in Monk/Unchained Monk then weapon focus unarmed strike, weapon specialization, etc and all those will apply to every monk weapon thanks to ascetic strike as well as all of your monk class features. If you take the weapon adept specialization you delay access to evasion for weapon focus and weapon specialization and trade stunning fist for perfect strike with all monk weapons.
Unchained Monks have access to some fun and powerful weapons.
there is the much overlooked gnome alternate racial trait called master tinkerer. it gives the gnome in question proficiency with any weapon he has personally created. so you want a shapeshifter that can wield a shapeshifting weapon? cool concept. i would make one weapon of each type, (thrown, bladed, blunt, etc.) and then enchant through creat magic arms and armor feat. give the weapons the transformative property. you crafted the weapon, and thus you are proficient with it, when it transforms... you still crafted it... win win
**EDIT**
AVR above refrenced the trait but did not go into depth, we had the same idea i just explained it
Brawlers can apply Martial Flexibility to any weapon they aren't proficient with.