I had been looking at throwable weapons, and really... there isn't much reason to pick anything over a throw axe.
Axe is 1d6, 20/20 weapon. Martial weapon proficiency. The only reason to not pick an axe is if the character does not posess a martial feat. Alike in the same vein, shurikens require exotic proficiency in return for even worse 1d3 of damage.
Personally I would suggest:
1. Axe is in right place, I would not change it.
2. Throwable daggers and darts to remain 1d4, but improved 19/20x2
2.5. Optionally to the above, keep darts as simple weapon 1d4 20/20 but upgrade daggers to 1d4 19/20x2, requiring martial.
3. Shuriken, I can see some use if it was 18-19/20, and/or x3 mod crit modifier to make it more distinct and a niche. Definitely not worth exotic perk investment otherwise.
4. Invisible Blade, heavily underutilized class would benefit greatly from having ability to count any throwable weapon as source of bloodstacks. Naturally keeping the prerequisite that throwables are size smaller than the user. I can already use crippling throw which is graphically a knife throw that nets slowdown and moderate bloodstack gain at a short cooldown. I should be able to throw regular knives for single bloodstack.
5. Make Slings also work for Invisible Blade, given it counts as Weapon Focus: Thrown.