Ive suggested this before but I didn't explain it properly so I will attempt to
Simply infection is a gamemode with a style matching COD. You start off as a human and after 20 seconds 2 of the players in the game will turn into zombies. The aim as a zombie is to infect all the humans, the aim as a human is to stay alive. To infect a human a zombie simply has to kill them and the human will respawn now as a zombie.
The maps are mostly COD based e.g. Nuketown. New maps could also be created, they simply require some hiding spots or parkour spots to give humans a slight advantage.
The levelling system (similar to that on cod) includes around 10 ranks between each prestige and 10 prestiges. Levels are determined by your xp level, this can be increased by getting kills, voting, or from kitxp. Levelling up improves human kits and prestiging improves zombie kits.
There are also kill streaks (Roflstomps) that offer: extra arrows, lighting strikes, gapples, stun grenades and wolfs.
Infection is an extremely addictive game mode due to the grind of going from level 1 prestige 0 all the way up to maxed out prestige 10.
I think this gamemode could be easily as popular
For more info here is a link to the plugin: https://www.spigotmc.org/resources/infection-deadlyfe-ranks-levels-powers-prestiges-kits-minigame.60827/
Not sure if its a working one but its the only public one currently available.