Color me skeptical. If it happens, the debate will provide some entertainment for political junkies, but the areas of disagreement between the two are actually quite small, and the debate is likely to focus on "soft" issues like being polite to women. Don't look for much debate on issues that get to the heart of the economic system.
The debate's primary significance is in how it marginalizes Hillary Clinton, but as an actual "debate" this event will be almost totally about slogans and style since the candidates already agree on many key issues.
Taxes and Spending
For example, part of Trump's popularity is that he has never suggested he'll do anything to significantly cut social benefits. Trump rarely mentions programs like Social Security, Medicaid, and Medicare, which comprise the majority of federal spending each year. Trump is, at the very least, a status quo candidate on these issues. Sanders, of course, wants to spend much more.
Health care may be one of the few topics on which there could be real debate, however, Sanders wants to expand government health care beyond Obamacare. Trump's position is to replace Obamacare with "Donaldcare" which will be "absolutely great." What we do know is that Trump on his own website has called for the expansion of Medicaid "to ensure that those who want healthcare coverage can have it." This in itself will drive up health care costs for everyone else, just as student loans increase tuition. On the other hand, Trump has called for some additional tax deductions for private spending on health care.
In general, if the topic comes up, the disagreement will be over how much more to increase social spending. Trump will want less, and Bernie will want more. In other words, Trump will position himself as "Bernie Lite."
The single largest disaster looming over the American economy — the massive debt and immense social spending (i.e., non-discretionary spending) obligations — will only be debated at the margins. Trump will say he'll somehow "renegotiate" the debt, but neither candidate has any idea of how to actually deal with the issue. And neither will support any actual cuts to spending.
In this case also, Trump is likely to want to increase taxes a bit less than Sanders. So, once again, it's a debate between Sanders and Sanders Lite.
Opposition to Trade
Both Trump and Sanders oppose free trade. In some ways, this has accidentally led to good positions for both men, such as their opposition to the Trans-Pacific Partnership and NAFTA. But both candidates oppose trade agreements for the wrong reasons. They oppose trade agreements because they oppose increasing trade. Both Sanders and Trump drank the protectionist Kool-Aid and thus believe that trade with China (and other trading partners) has "destroyed jobs."
Obviously, this position benefits them both politically, because it allows them to pander to union workers and related groups who are too ignorant or too self-interested to understand or care that anti-trade policy leads to a higher cost of living for everyone. Trump claims to be against burdensome government regulations, but he has no problem with regulating what Americans can buy and sell, or where they can send their money.
So what will they debate on this issue? Will Trump just spend his time demonizing foreigners who provide low-cost desirable goods to Americans? Bernie may instead focus on the need to punish and impoverish rural Asian workers (i.e. shut down "sweat shops") or impose greater environmental requirement on foreign nations. In either case, it will just be a debate over how much to increase government regulation. Again, we're left with Bernie and Bernie Lite. Or perhaps in this case, it will be Trump and Trump Lite.
Both Sanders and Trump are relatively good on foreign policy compared to Hillary Clinton, who is clearly the candidate most likely to start World War III. The problem here — for the purposes of debate — is that Trump has already disavowed Bushian foreign policy. Were Trump any other candidate, Sanders would lay into him for his support of the "disastrous" Iraq War. But Trump has already come out against the war. He even ran on the issue in South Carolina, which is possibly the most pro-military state in the Union.
In other words, Among the Republican rank and file, the Iraq War is regarded as a disaster, and no one is running as a defender of the Bush foreign policy legacy. So where will the foreign policy debate be? Trump may criticize Sanders's partisan support for the 2011 bombing of Libya, but Sanders will recall his tepid opposition to that move and explain how he didn't really support it.
But don't expect any talk of demobilization, closing foreign bases, or any significant cuts to military spending. Sanders has called for more "accountability," but even Sanders has refused to take a meaningful position against drone strikes, and continued spending that puts US military spending at a higher level than the next seven largest countries combined. Donald Trump's rhetoric on the need to "rebuild" the military, is alarming.
Worst of all, Trump declared his love of the police-state measure known as the "Patriot Act" when he announced he would attempt to stop private citizens from sending cash remittances to Mexico by using provisions in the Patriot Act. In other words, Trump is perfectly comfortable with using the cry of "anti-terrorism" to carry out domestic policy and further regulate private property.
Both candidates appear to favor fewer foreign invasions and large-scale military operations. That's certainly progress, and better than Clinton's foreign policy. But, again, it's hard to see what will be meaningfully debated on this issue.
Central Banking and Monetary Policy
Of course, the issue on where both candidates are especially ignorant is on the issue of central banking. Both Sanders and Trump have signaled their support for continued ultra-accomodative monetary policy, with Trumpdeclaring that the Fed should not allow the dollar to get too strong or raise interest rates very much. Sanders has criticized the fed for not lowering interest rates enough.
You won't hear anything in this debate about reforming the federal reserve, ending the dual mandate, or returning to the more normal interest rate policies of the pre-2008 world.
Focus on Your Feelings
What you will hear in this debate is a lot of talk about immigrants and diversity and religion (i.e., Islam) and what our feelings are about those things. Trump will score points with his followers by wrapping himself in the American flag (hopefully just figuratively), and Sanders will harp on the "social justice!" line for his followers. But, again, much of the dichotomy will be a matter of style.
Thus, there will be some disagreements about how we should feel about foreigners and the wealthy, but there will be precious little debate of matters that have have much to do with the ability of Americans to earn a living, start a business, or save for the future.What will be debated will have little to do with the massive inflation tax being imposed every day on holders of US currency which results from low-interest rate policies beloved by both Sanders and Trump. The contentious issues will have little to do with the need to de-mobilize the US military and bring the troops home.
So, if it's entertainment you crave, the "debate of the century" may have some amusing zingers. But it won't have much more than that.