One of the things I loved about later editions of the game is the ideas of templates. It allowed you to come up with all sorts of strange and interesting ways to alter a monster without needing to create something from whole cloth. I’ve mulled the idea over and come up with a quick way to modify existing monsters to make them a bit more special and potentially dangerous without having to create new ones each time.
http://micahblackburn.wordpress.com/2013/06/14/house-rulesmonster-templates-osr-style/