Tax has become quite a non-factor in this game for a while.

As you have it already implemented it would be great if you make more use of it. Some proposals:
- bring in a government dependent tax-factor multiplied to the set tax level (e.g. people more strictly adhere to paying taxes because they are forced to)
- restrict the allowed tax-level for some government types e.g. for a Republic only a tax-level between 60-70% is allowed (just as example);
- bring in a time- or landsize-dependent tax factor multiplied to the set tax level (e.g. the longer you are leader the less willing are people to pay taxes:P; the larger your country is the less oversight have you got over your taxpayers;-)
- the tax-level might have influence on your warring readiness or your army strength (you do people something good by lowering the taxes, they are more enthusiastic to fight)
- tax-levels might effect the productivity
- simply adapt the formulae to make the selection of the best tax-level more situation depedent

whatever ideas you have.:)

Zero tax for the demo is one of the most powerful perks for a government.

That's market commission, not income tax, koon.

Tax is definately a "set it, and forget it" option that needs only be looked at once or twice.

An option might be to have the tax rate affect the frequence of printing money/tax rebate events. 35% may be optimal for income, but it might not be optimal for political unrest. A lower tax rate then would trigger the random event less often, but it would be a statistical trade off that you were taking a chance rather than setting a single value for a single parameter.