Three things off the top of my head:
1) Accelerate efforts to retire/modernize
legacy IT systems, particularly those that are now as old or older than some folks on this forum.
2) Depoliticize the tax code. One of the primary drivers of building efficient systems - whether human systems to technological systems - is to reduce complexity. The tax code changes with political tides and has become a method of driving social change and/or incentivizing (positive or negative incentives) personal and corporate behavior.
A tax billing and collection system should be about one thing: tax billing and collections. Decades of efforts to make it something it was never intended to be only hinder the IRS's mission, make creating efficiencies in the system extremely difficult, make life harder for IRS employees, and hinder efforts to modernize systems.
The other way to say this: I should be able to wake up on January 1 each year, and (based on my income) have a reasonably good idea of what my tax bill
for that year will be. Most Americans (including business owners) wake up on January 1 and sometimes don't know what their tax bill
for the prior year will look like. We've gotten so used to living like that, we fail to recognize just how absurd it really is.
3) Once one and two have been accomplished, the IRS gets much less expensive and more efficient to operate. The size and scope of the organization can be reduced. Simplification of the tax system could make projecting income and expenses for individuals and corporations MUCH easier, and as a bonus, reduces the frequency of tax avoidance schemes that require enforcement activity. It also seems likely that a predictable tax could have the effect of increasing economic activity (which increases tax revenue).