This is a great idea.

In addition to this we should be able to specify which skill icon and associated tooltip and cooldown are displayed on the face of the macro button.
Using an example that will be familiar to some;

#tooltip Skill3
/ac Skill1 <t>
/ac Skill2 <t>
/ac Skill3 <t>

This macro would, as normal, always use the first available ability from Skill1, Skill2 or Skill3. The button face would always display the appropriate icon, tooltip and cooldown information from Skill3.