More boss Infected would be a nice idea the trick is to add somthing to the gameplay, you should add new weapons though if new infected have other abilties like say these ones me and friends ended up making in our conversations during Vs mode thought I would share with you all our findings.
Howler- The Howler takes on the apperance of a human with a frog like throat and his bellowing alerts people of his arrival when he gets close enough he screams at you causing the game to mute itself for a short period. Now the Survivors really do need to work together as they wait to be able to talk again.
Buzzer- This infected takes on the apperance of a strange mutated insect (Think The Fly) it buzzing noise warns of its close arrival but its deadly due to the abilty to fly above the survivors and bring death from above. Obviously this flying abilty won't last for long and then they will need to recharge it. The survivors now need to think about possible air attacks on top of the normal directions assoicated with the Infected.
Pyro- As you might have guessed this one is a walking flamethrower, its gargled voice (which oddly enough sounds like someone wearing a gasmask...) annouces it's arrival before spewing forth a blaze of fire at the survivors at the cost of its own health. The Pyro never lasts long but he doesn't need to the fire he brings forth can easily throw any plans into disaray.
Spider- This crazed infected has elements of an insects armour poking forth and strange tendrills coming out, the Spider can climb on walls and celings but is the opposite of the Smoker. Where as the smoker drags away the Spider forces its prey to stay in place by spraying an adhesive that causes the survivor to stay glued to a spot for a short time It then can attack the helpless survivor.