Crank + Infected/Zombies + Aliens. That would be for a fun game. The story would be Aliens came to earth and infected the humans. The Infected humans would be zombiefied and once infected would join the Aliens to infect the humans.
If the infected doesn't comply they will blow up within a certain time limit. Once an infected gets a kill every infected players time limit increases for a longer life. No respawning in unless the infected has a tactical insertion down, but the infected will not respawn in even if they have a tac. down if their time limit ends and blows up. They will have to wait until the end and spectate.
If the humans defeat the Aliens and the infected they win. The Aliens have a time limit they spawn in. At beginning theirs always an infected, so that should stay the same,(Have an alien appear on one of the players heads and that players infected), Game starts.
Well that's what I think could improve Infected.
By the way that video of you playing sucks(your voice echos in the video), but it looks like you're having fun, guess that's what counts.