A few thoughts:
Two of the titles suggested in "Untitled Project I"'s title thread seem like very good choices for these projects: "Heresy" and "Häxan".
Heretic and Hexen have pretty contrasting atmospheres, with Heretic quite vivid and bright and Hexen pretty gloomy and dark. What if the open content project for Hexen was aesthetically Diablo-influenced, i.e. Copperplate Gothic text, lots of black and red, etc. (This would fit well if you're planning on using pre-rendered sprites, too). I don't know if Eriance would be willing, but if so, these potions would be a perfect fit: http://forum.zdoom.org/viewtopic.php?f=19&t=15999&p=373733#p372232