Intimidativeness - Besides the Intimidate skill, Strength, character size modifier (big dude = more intimidating, small dude = less intimidating), and HP points would be factored too. Even, let's say, if a strong axe-wielding level 30 Half-orc has spent 0 points on Intimidate, why wouldn't it be intimidating?
Persuasiveness - Charisma and Bluff (perhaps Perform too) would be factored, together with Persuade, on this stat calculation.
Putting points on Intimidate and Persuade skills would still remain as the best option to increase their correspondent stats. These changes I proposed will only increase these stats to a certain extent, otherwise I believe people would simply stop spending points in those skills.
To illustrate what I mean:
How can this killing machine not have an extremely high intimidativeness? Just because it has no points in the Intimidate skill? I mean, Raastred is a huge minotaur with an acid-drooling greataxe.