I love these armchair opportunities for college-level challenges without the admission fees, I'll bite. Step 1: disallow corporation as a legal taxable entity, private ownership only. Step 2: Subtract expenses from income. Step 3: Divide by 10. Step 4: If more than zero, send a check.
"Fair and efficient" in terms of opportunity is easy. In terms of outcome is a fool's game IMO
But what about investments and assets? A car that you bought for the company has a limited lifetime. So its value is slowly decreasing, should that be considered an expense?
What about R&D? We want to incentivize it, so that companies do more of it. Etc., etc.
As far as I can tell, all businesses would like to be able to deduct all cash outflows whenever they occur, regardless of whether it's capex or opex. Businesses depreciate across long time cycles because that's what they're forced to do.
As long as they can carry forward losses, don't all businesses want accelerated depreciation?
Everyone thinks that they can come up with a much better tax system, and they always start with something super simple that meets their tax needs. But a simple system that meets one person's tax needs is usually not so simple, fair, or efficient for other people. And when you take all their needs into account when you redesign the system...you end up with something pretty close to what we have right now.
For example, try designing a tax system that is both fair and efficient with respect to agricultural income and tech income (this is a fairly common assignment in university tax programs). It won't be simple. And that doesn't even get into how to reconcile the tax treatment of business income with the tax treatment of individuals, or the many ways they earn and spend income, or the complexities of other types of businesses (like manufacturing, retailing, resource extraction, in-person services, etc.).