BRUSSELS—A confidential document debated by eurozone finance ministers detailed for the first time what Greece’s creditors could do to ease the country’s debt load and how that burden would develop over the coming decades without new relief measures.

The document, which was reviewed by The Wall Street Journal, served as the basis of Monday’s emergency meeting, in which the ministers discussed for the first time the possibility of further debt relief for Greece. The finance chiefs said they aimed to reach a deal on the matter by their next meeting May 24 to resolve an impasse among the country’s creditors and release much-needed bailout funds to Athens.

But the wide variations in the document’s debt projections signaled more difficult discussions ahead among Greece’s lenders over how to allow the country to stand on its own feet again after three international bailouts. Those creditors include the 18 other eurozone countries—led by Germany, which opposes all but minimal changes to Greece’s debt burden—and the International Monetary Fund, which wants to see as much relief as possible.

Under all but the most optimistic scenarios, the document pointed to serious concerns over Greece’s ability to repay its debt, which stood at 176.9% of GDP at the end of last year. The results of “this analysis point to serious concerns regarding the sustainability of Greece’s public debt in the long term,” the document says.

Without debt relief, annual interest and principal payments would be far above what is generally considered sustainable, the document says. However, the big divergences in the scenarios give ammunition to both supporters and opponents of aggressive debt relief.

“Today was about opening the debate, exploring options,” said Jeroen Dijsselbloem, the Dutch finance minister who presided over the meeting.

But he conceded that without relief, Athens would likely never be able to repay its debt, the highest in the eurozone by far. “My assumption will be that there will be a problem of debt sustainability that we will need to address,” he told reporters.

The variations are due to differing forecasts of how much Greece’s economy will grow in the coming decades and how much money it can put aside to pay down debt. Under the most pessimistic scenario in the analysis, Greece’s debt could rise to a staggering 258.3% of gross domestic product by 2060. The most optimistic scenario, meanwhile, would see debt fall to just 62.8% by 2060 without any debt relief measures.

The document, prepared by EU institutions, sets out what measures are on the table. The suggested steps don’t include a reduction in the principal of Greece’s loans from the eurozone, which total almost €200 billion ($228 billion).

Instead, the document suggests eurozone countries extend maturities, limit annual repayments and cap interest rates, along with other measures. In total, those measures would reduce the country’s debt-to-GDP ratio by 31.2 percentage points.

The measures proposed in the document mostly focus on €130.9 billion in loans given to Greece under its second bailout from the now-defunct European Financial Stability Facility.

Average maturities on these EFSF loans should be extended by an average five years to 37 1/2 years, the document says. Annual payments on the principal of the EFSF loans should be fixed at 1% of GDP until 2050, while interest rates should be capped to 2% of the loans until then, the document says. Any outstanding debt and interest payments would then be split into equal installments to be repaid after 2050.

In addition to the changes to the EFSF loans, the document suggests two other measures. National central banks in the eurozone as well as the European Central Bank should return some €8 billion in profits from Greek government bonds to Athens.

Greece should also be allowed to use any leftover money from its third, €86 billion bailout to repay early loans from the IMF. IMF loans carry higher interest than money borrowed from the eurozone bailout fund.

Greek Finance Minister Euklid Taskalotos said moves to reduce his country’s payment burden could help its depressed economy to start growing again. “It’s a great relief to have this debate,” he said. “We should be working toward creating a situation where at last Greece can turn a corner.”

Still, much work and pain lie ahead before any relief would come. Greece’s Parliament would have to pass a mechanism to trigger further savings of 2% of GDP, or €3.6 billion, in case it misses the budget targets in its bailout program. That comes on top of €5.4 billion in austerity measures, the bulk of which lawmakers approved Sunday.

Mr. Dijsselbloem also said that even if eurozone finance ministers agreed to debt relief now, it likely won’t be implemented until the bailout program expires in 2018. That is meant to ensure that the government in Athens is actually committed to keeping healthy finances.

“Whether it all is going to be substantial enough, no one has given a verdict yet,” he said.

The fact that Monday’s document was prepared just by the European institutions overseeing Greece’s bailout—without the Washington-based IMF—highlighted a larger dispute between the two sides.

The IMF, which co-financed Greece’s first two bailouts, has consistently had more pessimistic forecasts for Greece’s debt ratio. It has refused to lend Greece any more money unless the country enacts more austerity to ensure a high budget surplus or eurozone countries write down a sizable amount of what Greece owes them.

The differences stem from divergent forecasts for economic growth and Greece’s primary surplus, which doesn’t take into account interest payments, as well as how much it has to pay for raising money on financial markets.

The European institutions believe that, assuming Greece fully implements the terms of its bailout program, its debt would peak at 182.9% of GDP in 2016 and fall to 104.9% of GDP by 2060, according to the document. The proposed debt relief measures would bring it down to 73.6% in 2060.

However, even small moves away from the bailout program’s targets, including lower primary surpluses and higher interest rates, would quickly push the debt back up.