Idea: There should be a unit that has both melee and ranged attack.

Musketeers have this in AoE3, where they will shoot their rifles by default but grapple with their bayonet when enemies get too close.

I think AoE2 could have a slightly different take on this concept. Cavalry that is strong up close but weak from range, you can toggle a button to make them fight with their swords at close range or fire their arrows from afar.