Here's a very simple variant that adds a lot to multiplayer games.
During an attack, the attacking player, or any player that can assist or interfere, can discard plot cards in order to affect the odds of success (up or down). Each discarded plot card can count for 1 point of power.
This has the following benefits:
1) You can trade favors with other players more easily. In the original Illuminati game, a lot of the game was in trying to garner assistance (or discourage interference) from other players. INWO makes that harder because there is no money. Now Plot Cards can be used in place of money.
2) If you're playing with a common deck, you can get plot cards that aren't too useful. Now you can at least consider them worth something.
3) There is more strategic decision-making in a "battle". Not just whether to assist with your other groups, but whether to discard potentially useful Plot Cards for the sake of better odds.
I imagine each Plot to represent not only the plans for an action, but also the resources to carry it through. Thus discarding the plot can be considered "redirecting" those resources. Also, it seems fitting that the Gnomes of Zurich (aka the bankers) can hold an extra Plot Card (i.e., they can store more resources).
One other thing. Because Plot Cards are now intrinsically more useful, the Network's special power of drawing 2 Plot Cards each turn should be balanced out by reducing their power by 1 point.
In my experience, there is much more room to negotiate in multiplayer games when there is a fluid currency.