One noticable thing about the Game Worlds is that the players are often spread out thin amongst the Game Worlds.
If there was for example a periodic “10% Bonus XP to Zombie Massacre For the Next Hour,” then one would be rewarded for playing the Game World at that specific time, which in turn will hopefully make more players play with each other.
Maybe not do it too much, to make it like a small daily/bi-daily event, so it would be a special occasion. In addition to revolving what games are given a bonus, so players may be more motivated to check out Game Worlds they don’t usually play.