Slitherine

If you had the ability to fire volleys of pilum/javelins/ or bows at a certain spot in the line of your approaching enemy that could give you a strategic advantage, yes?no?

Say you fired at the outside flanks you could frighten your enemy, so they bunch together in the center. Once this is done you could order your infantry in a let loose on a enemy that operates in a loose formation. You could use this to your advantage and tell your archers to rain arrows down upon the crowded mass of men.

Say you fired into the center of a line, your enemy might bulge outwards thinning its entire line. Once this occurs you have your infantry/calvary rush the center, and you snap the enemy line in two, on contact when your infantry/ calvary rush it. All the meanwhile the flanks are curling inwards, thus you open up four new flanks upon your enemy's battle line.

You must first convince your troops that throwing their missiles against the ground is better than against the approaching enemy.

I think the concept here is "killing zone", and you must wait until gunpowder era to see this happening (better still, till ww1).

I'm sorry I do not understand what point you are trying to make with this comment "You must first convince your troops that throwing their missiles against the ground is better than against the approaching enemy."?

If you are talking about killing zones, then the concept was alive. The legionaires would wait to hurl their pilum, so would the javeliners.