There should be upgrades such as expansion of your empire(I.E. More spaces to build your buildings) This will make the game less repetitive. The max level limit of building should not be 9 as it is possible to get all your buildings to 9 in just one day. Maybe add an upgrade that allows you to go beyond level 9? Also, if you want to make your people happier after taxing them highly, you should decrease your tax rate to below 50%. (If the tax rate is set to 30%, the empire happiness will increase by 1% for every 10-15 turns you spent on increasing your money.)