well, I admit that there are times you can play a game and it seems easy, but there are times you can play it and it is rough. The game seems to have a slippery slope aspect. When you start loosing it gets a lot harder to bounce back. The biggest thing to me that makes a single game harder then the next is the boss on the board, so an expansion with harder bosses would help
Now without an expansion possible ideas for making the current game harder....
1. all doom track spots on a boss card release a monster.
2. take out all the adventure cards that are worth 1 point.
3. ignore special abilities on character cards, or maybe remove some of the more powerful characters from the game.
thats all i got for now.