My two regular gaming partners are out of town so I'm tossing out this variation without a present ability to test it... Seems it just might keep them attentive when it's not their turn, and even might help them to plan ahead.
First, throw out the rule allowing each player three dice rolls on his turn.
A token is required to identify PLAYER ONE for every round, as in the governor role in Puerto Rico.
PLAYER ONE starts the round by putting dice into play, equal to his number of cities. Each player in turn, including himself, then adds or removes dice according to his own number of cities, may reroll the non-skulls once, and then scores his own sheet.
For example... suppose PLAYER ONE has four cities, PLAYER TWO has five, and PLAYER THREE has three.
First, PLAYER ONE tosses four dice (equal to his own number of cities) to get the round started.
He tosses one skull and three goods. He must keep the skull, and he chooses to re-roll two of the goods. He rolls another skull and a worker. He then scores his sheet.
So now the active dice for the round are two skulls, a good, and a worker. PLAYER TWO adds one die into the round, bringing the total to five, equal to his own number of cities. He must keep the two skulls, but decides to roll the other two along with his newly introduced die. He rolls another skull, a worker, and a food. He then scores his sheet.
Now there are five active dice in the round (3 skulls, a worker, and a food), but because PLAYER THREE only has three cities, he removes two dice of his choice. He removes two skulls, leaving a skull, a worker, and a food, and he elects to not re-roll anything. He then scores his sheet.
The governor token is then moved to the left and a new round is started.
So, what do we have here?
Wildly different strategy and fewer dice rolls than the official game, to be sure, but a lot more interaction and probably a lot more attention to the dice when it's not your turn. Also, inasmuch as PLAYER THREE in the above scenario had fewer cities and could toss out dice he didn't want maybe there would be less of a rush to collect workers to add cities?