I have a feeling the idea possibly won't be received well, but I may as well suggest it It could probably be built upon the invasion gametype

It wouldn't be a regular game of UT that requires a lot of skill, it'd be a lot more chilled and also more accessible to more people who aren't so good at the game.
You've probably seen it in a few other games, one person starts out as infected, and the goal is to infect everyone else by touching them (could have an extra animation for this)

It would be highly customizable, so you can change various things about the infected/survivors, here are a few basic ideas:
Survivor kill points - how many points you get for killing an infected
Infected kill points - how many points you get for infecting someone
Infected knockback amount - how much weapon knockback will affect the infected
Distribute points (true/false) - if there is more than 1 point per kill, distribute them based on the percentage of damage players have done
Survivor/infected base stats - health/speed bonus/invisibility
Maximum infected - infected will respawn until this number is hit, once all the infected are killed, you move to the next round

This would allow a variety of games, like having lots of super fast weak infected swarming you, or slow strong ones that require teamwork to bring them down.

It would definitely need an incentive not to camp (possibly health degeneration if you don't keep moving through the level), but it is just a basic idea currently :P