individual initiative is bad when there is 27 + enemy
we would need "group" initiative
Please explain how increasing potential output of a party by 50% "make[s] the fight longer and more tedious" when the number of enemies stays constant.
Why do you have to increase the number of enemies? If the reason is because "this would make most (current) combat encounters too easy" then modify the encounters (increase difficulty) without increasing the number of enemies.
This is 'DM has to adjust campaign to accommodate an additional player' basic 101. ...Strahd is gonna get 10% more HP and now does 1d10+5 damage with his main (unarmed) attack.