Get away from obviously failed and uninspiring Neoliberal policy. Adopt a platform designed to address core, chronic issues of the laboring class writ-large:
1) Healthcare reform: implementation of a universal single-payer healthcare system
2) Large-scale student debt forgiveness, implementation of stricter regulations on student loan providers and for-profit universities, implementation of programs to make college more accessible and more affordable (subsidies to prospective students, more and better access to grants, etc.)
3) Large-scale infrastructure improvements and/or plans to fund, maintain, and expand public transportation nationwide.
4) Implementation of the Green New Deal
5) Laws designed to protect and expand unions and union representation. We're in a spot where "unfettered" capitalism is broadly recognized as an ill. Union is perceived as a dirty word due to 40 years of Neoliberal propaganda and cultural conditioning, but I think it would be fairly easy to adopt a Soc-Dem type countermessaging that places unions as a necessary and healthy counterbalance to corporate tyranny that could be met with broad-based support. It's an easy answer, e.g. to complaints of outsourcing, soul-sucking retail work, abrupt factory/store/office closures/bankruptcies that result in large severance packages to corporate leadership and nothing for the actual workers, etc.
6) Greater regulation of
a) Gig economy jobs
b) Timesheet manipulation of hourly jobs to avoid full-time benefits
7) Voter expansion and protection. The bill the Dems introduced this week (mandated 14 days of early voting nationwide, election-day registration, election-day established as federal holiday) is a credential which should put at the front of their 2020 campaign
8) Stricter oversight for police departments
9) Comprehensive immigration reform, including a path to citizenship for DREAMers, stronger protections and avenues of access for refugees, broader and easier access for individuals to immigrate into the country legally
10) Stronger federal protections for access to abortions
11) A backstop rule that allows for continued government funding in the event of the failure to pass a budget (e.g. government reverts to previous year's budgetary allocations)
The Democratic base historically has been a coalition of women, minorities, educated urban middle/lower-middle class, and organized labor. The adoption of the Neoliberal ideology under Carter, intensified under Clinton precludes any meaningful appeal to, essentially, any of those groups. All the party can do is pay lip service to them while doing nothing to actually help them when in power. At best the Democratic coalition sees the occasional bone thrown their way and at worst they see their political power actively undermined (particularly in the case of the unions). This is why the Dems failed so catastrophically in 2010 and 2016. The components of the Democratic coalition can only be duped so many times before they recognize that the Democratic leadership doesn't actually represent the base's interests. This was especially visible in the form of Hillary who
completely ignored the African American and organized labor arms of her coalition, and this was repaid in the form of the "shocking" losses in Wisconsin and Michigan, both states with historically strong Union membership.
By contrast, Obama won in 2008, essentially, by playing the part of a populist and pushing a message that promised meaningful change for all the apparently disparate groups of the Democrat coalition. "Change" is a powerful message for women, minorities, the urban poor/middle class, and organized labor, all of whom have spent the past 40 years getting screwed over by the Neoliberal policies of both the Democrats AND the Republicans.
All of these policies are broadly popular, and more to the point, predicated on common-sense solutions that can be messaged simply and repeatedly. Trump won, in no small part, I think, because his messaging was: a) simple to convey ("Build the Wall", "Drain the Swamp", etc.), b) easy consume and understand, and c) possible to repeat consistently without distortion. The Democrats sit astride a bunch of policy positions that are very easy to explain and message and resonate with a broad base of voters. They have just been historically very, very bad at messaging them. Less "I'm with Her" and "When they go low, we go high" and more "Health care for all," "Save the planet," and "Fair treatment for workers."