I believe that the Arkham Horror Game would be improved thru the addition of more plot elements and cooperative play. One idea that we have been experimenting with are cooperative play cards.

We apply these rules during upkeep phase:
1. Purchase Cooperation Item
a. Cooperation cards represent an opportunity for players to help other players during gameplay
b. Cooperation cards may be purchased during the upkeep phase
c. A player may draw and, optionally, purchase one cooperation card each turn
d. A player may only hold one cooperation card during play
e. Cooperation cards may be purchased as follows: i. Discard 3 toughness worth of monster trophies to draw one card OR; ii. Discard 1 gate trophy to draw one card OR: iii. Pay the dollar value indicator on the face of the card
f. Cooperation cards may be played and discarded at any time to assist another player or a Citizen in the same location on the board

I would be very interested in feedback.

Card images attached.

Not sure I managed to post images properly.

here is the link

https://www.dropbox.com/s/7cj854xa7ukm8nm/Coop%20Cards.zip

It's a neat idea, but I think in most cases I'd probably just play with Relationship Cards from Lurker at the Threshold instead. That one provides cooperative bonuses between you and the person next to you and is given freely at the start of the game instead of acquired in game via costs.