Know what's one of the hardest development tasks for a GURPS add-on -- heck, for just about any RPG? A slick system for variable action times. That is, rules to allow one character to perform certain actions (typically attacks) twice as fast as another, or five times for every four of a foe's, etc.

HERO's Speed stat doesn't quite have it. It's great that Speed 10 lets you have an action rate a little slower than Speed 11, which is a little slower than Speed 12. . . but what about a creature a little slower than the human norm of Speed 2? The only level allowed below that is a full halving of Speed to 1, and then no action at all (Speed 0?) below that.

The ideal system would interest itself in relative differences, equally allowing for battles between psi warriors (typical attack times: 1/10 second, 1/15 for the fast ones) or between the living mountains of Ryol III (typical attack times: 3 Earth years for a "punch", 2 for a "jab", 1.5 for a "fast" mountain's jab, etc.).

I've yet to see a proposal for GURPS that makes a good start (IMO). I certainly don't have one myself.