Personally I think agility could possibly be worked into being a combat skill, one that you don't actually train in combat much like prayer. Basically you could have special "dodge" moves that give you a chance of avoiding an enemy's attack, moving quickly behind them and getting a chance for a free hit, although with a chance of failing. It sounds sort of awkward in the way I'm picturing it to be honest but I think it could be worked into play somehow. Either that or they could have it improve the attack speed of any weapon as it's increased, although not by a great deal but significant enough when you're at high levels. Agility is one of the skills that needs to be greatly expanded on in my opinion. There's just not enough incentive for most people to train it, hence even the higher level people in the game typically still have quite low agility. It's just not currently rewarding enough...unless you're obsessive like me.