For larger casinos (doesn't make sense in the small 1-star gambling halls), perhaps the ability to introduce guest rooms into the casino? The design for the rooms could probably be doubled with the condo design (with some extras, like bathroom fixtures, mini-bar, etc.), and chips could be gained whenever NPCs decide to stay in the rooms. Maybe also add housekeeping and room service jobs with it?

Right now, there's no good incentive to go from 3-star to 4-star because it's just one more floor, but perhaps an exponentially larger number of hotel rooms could be the incentive to push people to it?