The Hole is a great mechanism to open the knife.
The only improvement I could think of would be to put a bezel ring or concave thumb pad in the hole. The ring or pad would rotate in the hole as you pressed to open the knife. Since the ring wouldn't move against your thumb, like the hole by itself does, the ring could be textured or knurled for better purchase.
The only disadvantage to this is ease of cleaning. The round hole only needs wiping out with a cloth every now and then to remove oil and dead skin (common wherever you have a part that contacts and rubs against the skin). With a bezel ring, the ring would have to be removable to facilitate cleaning, otherwise it would be a major pain, and if enough grit got between the ring and the hole, the entire thing would jam, worse than having no ring at all.