I used to use a CoC card coffin as the monster cup for Arkham Horror. Now, with all the expansions it just is not big enough. I think a monster cup ( or at this point, "bowl" or "barrel" ) with a few bonus monsters would be great.
Also, I think a set of adventure or plot cards for a specific storyline would work well. Some of the criticisms toward the game is that the cards are a series of random events. A set of cards which can be utilized by the investigators which moves the plot along and gives them some sort of bonus. An example might be a Cthulhu scenario where if the investigators spend 3 clues at Arkham Asylum they get the next plot card which gives them an item or ally and directs them toward the next plot card in the Old Woods.
I do understand that I just said that Arkham Horror could use more cards. Clearly I've failed a sanity skill check.