So you want to have 5 Melee Minions and 1 Caster? Okay
Let me explain to you why that would be a bad idea.
Non- AD Champions. Or for clarity sake Mages.
Have a timer on their wave clear. Its their cooldowns. They are inhibited from doing damage as long as their spells are on CD.
Melee Minions have higher defensive stats that continue to get higher and higher as the game goes on. That then hurts all champions that are 1 Not AD or 2 have to wait for CD to kill minions effectively.
This doesnt even get into current baron buff.
Current baron buff makes MELEE minions take 75% reduced damage. On top of the MR and Armor they also recieve. Making them time consuming to kill for the enemy team. Thats at 3 Melee's. You are increasing the time it takes to kill that wave by 1.66. When they are not taking harass. When they are taking harass from the enemy team you are effectively making the time it takes to clear that wave jump at least 2-3 times higher.
Ranged Minions give some form of counter player to when a range champion attacks a melee champion. The Ranged Minions just switch aggro and start attacking the Ranged Champions. Where as the Melee have to move over to that Ranged champion. You would inadvertently make the ranged champions in the game even stronger by changing minions to this.
Ex. Lets say you are against Kennen at top lane. His auto does about 50 Damage to you at level 1. But he only recieves about 13 damage per Caster. 3 Casters do 39 damage. You only took 11 more damage then he did before he entered a brush to force the minions to drop aggro on him. (If he doesnt then he took more damage then you saying only a 4th caster attack lands 52 damage).
Lets apply your change then Reverse the senario.
Master Yi Q's and Auto's Kennen in his wave.
Kennen walks back while Yi is chasing.
Master Yi gets 4 Autos off while running at Kennen. (say this takes 3 seconds)
In that time since the first auto from Yi he has taken at least 10 autos from Melees that do 15 damage each? Give or take? + 5 Autos from the Caster.
So you have taken 150+ 65= 210 damage from minions in 3 seconds. Without Kennen even auto attacking you back or using a spell. To your comparative (at level 1) 290 Damage w/o armor damage reduction which at a base is about 27% reduction. You dealt 211 damage to Kennen. Congrats you beat him by 1 damage when he didnt do anything at all to you.