Democrats will almost certainly not have a 60 vote majority in 2021, because getting to 60 seats would require going deep into GOP territory, winning multiple races among states like Kansas, Kentucky, Nebraska, Alaska, Mississippi, South Carolina or the like, which would require an LBJ 1964 level blowout.
So a big issue for the coming election is what the candidates think should be done about the filibuster. Almost none of the stuff people are talking about as their litmus tests will go anywhere legislatively if the GOP retains the ability to use the filibuster to block bills indefinitely. Basically the only things that could pass would be tax rate and budget spending changes, using reconciliation. Medicare for All, criminal justice reform, environmental regulations, labor reforms like mandatory family leave or union rights bills can't pass that way. So it doesn't matter if you want $15 minimum wage or $12, single payer or a public option, an end to cash bail or marijuana legalization, etc. if Mitch McConnell has a veto. What exact reform is pushed could be finessed to pretend they're not really getting rid of it (for example, returning to the requirement that you actually be giving a speech during your filibuster), but the implicit 60 vote requirement to pass legislation needs to go.
Obviously, the president can't eliminate unilaterally, but they could have real influence. This will be an important issue to push senators on as well, of course.
‘Everything stays on the table’: 2020 Dems weigh killing the filibuster - POLITICO
So in this regard, Warren is out in front of all the other contenders. Gillibrand and Sanders expressed some ambiguity on the issue and are perhaps open to the idea, while Cory Booker explicitly said he's opposed. Booker wasn't my top choice in the first place, but I'm generally going to rank any candidate explicitly open to eliminating the filibuster above any candidate explicitly opposed to it.
To the point that if Bloomberg and Biden came out for eliminating it and Sanders and Warren said they would fight to keep the filibuster, it would scramble my evaluations of the candidates. I don't think that's going to happen, but that's how important it is as a question, IMO.
The next most important after abolishing the filibuster is a voting rights and election reform bill like HR 1, DC statehood and a binding PR statehood referendum (which adds even more wiggle room for big legislation). Democrats need to stop accepting rules and distributions of representation that rig the game for Republicans.