Addendum: make filing taxes a largely automated task where data are shared with the government so the online submission is merely a review of what is already on your W-4 (or what have you). Make it so dead simple it should take no more than 15 minutes for the average citizen with a computer. Anything that is purchased or paid to you should already be built in: if you paid mortgage interest, the deduction is automatically applied from data supplied by the lender; if you installed solar panels on your roof, the vendor will have already informed the government and the correct deduction is pre-applied; if you registered a corporation in your name, the filing service will relay your costs to the government, as will your lawyer, if you did not file personally. If you paid interest on your student loans, the taxman will know long before April 15.
Tack on a processing fee for manual deductions that scales with the potential cost of an audit.