Haki changes the people around you.
Devil Fruits change you/the world around you.
Both are a result of your personal reality overtaking the world around you.
Luffy will surpass both Imu and Roger, in that Imu's Haki couldn't compare to Joy Boy's, and Roger didn't have a Fruit.
Luffy's Haki, his ability to change people around him, combined with his Fruit being able to physically alter reality, will allow him to truly change the world.
Why do Kuzan and Garp have strong Haki, while most Marines don't? Because, they put their sense of self, their true justice, above the WG.
Kuzan refused to kill Saul or Robin, and helped them survive. Garp has stuck his middle finger up at the CDs/WG for years.
What makes Luffy one of strongest fighters in the series, is he refuses to compromise himself for the sake of others. That was the real point of Usopp's recent speech: Luffy is strong because he follows his own convictions.
Those with the strongest Haki are those who are most honest with themselves.