Creating new creatures.

Maya 6 is a product from 2004, I believe. And it should be quite obvious that the creators of Heroes 5 couldn't have used a Maya version younger than 2006 (the year the game was released). More likely, it was version 2005, because graphics aren't created in just a couple of months. And anything newer won't work. Version 6.5 might still be possible, but anything released later definitely won't.