If you own Arkham Horror you should go straight out and pick up a copy of Battlestar Galactica the Board Game. BSG shares a lot in common with Arkham in that:
• It’s a co-op experience which will appeal to your Arkham group as well as help pull in new players (along with its Sci-Fi element);
• Variable characters for players to choose from;
• Lots of potential for player-player interaction in terms of figuring out the best course of action;
• The Crisis Cards drawn at the end of each player’s turn acts like a hybrid between a Mythos Card and a Rumor.
However the major difference between the two games is perfect for those who have always wanted to Join the Winning Team. The Loyalty card given at the beginning of the game and half way through allow for some players to be Cylons who attempt to thwart the Human players. This can be done through in-game mechanism or by meta-game cross-talk between players such as urging the use of more resources than necessary or redirecting players to a good but not best position on the board.
In a six person game this can result in two-teams of 3 being formed allowing retention of the co-op element but adding a competitive one as well. I think it is this last point that makes BSG such an attractive game to all types of players new and old and a favorite among my group.
I know this is an odd post, but I think that Arkham players would be particularly drawn to the game and “get” its type of play but might be sitting on the fence about investing in another game. Do it. You won't disappointed.