I think more ways to implement taxation would be welcome in game, to make it more interesting, spread the burden around a little more and make it more possible to eliminate certain kinds of taxes altogether, or at least reduce them substantially (I’m thinking income taxes here). Also alternative ways to tax the same thing.
-One mod from D3 introduced a land value tax as opposed to property tax. This also reduced the real estate bubble problem but is less onerous than the property tax, as the land value tax is on the unimproved value of the land, which makes the affected groups less angry (and raises less money).
-Prostitution Tax after legalizing sex work; should be profitable!
-Casino/Gaming Tax after legalizing gambling
-Windfall profits tax (on lottery winnings for example, game show winnings, and/or unusually large profits of corporation); also, introducing a federal or state lottery would be interesting!