In Ilomilo, you can get unlockable Keflings and Raskulls costumes for Ilo and Milo by purchasing the other games. Additionally, "You can unlock ilo and Doug as playable characters in Raskulls," says Raskulls dev Halfbrick, "and also unlock Raskulls characters within ilomilo and World of Keflings by purchasing the full range of games." And buyers of Raskulls or Ilomilo can build homes for characters from those games in A World of Keflings, according to a statement posted by Kotaku.
Even crazier: you can actually get Raskulls early by buying A World of Keflings. Raskulls can be bought from within that game, a week before its December 29 release. That's two out of the three games available early!