Once the Democratic field for the 2020 primaries is set, there is something that I would like to see all of the candidates do as a unified front: release a statement of principles and intent that all of them agree on as a way to make crystal clear to the public what the alternative to life under Pence (ahem) would be no matter which of them goes on to be on the ballot for the general election. And keep the DNC out of it...let this be a joint product of the candidates themselves. It can consist in part of what the candidates repudiate or stand against regarding the Trump administration or the GOP and its adherents but it should, as a statement, consist mostly of what the Democrats plan to do and the principles that guide them to do it.
No matter which of them wins the primary, some of them will go back to being senators and will still have a role in bringing about a shared agenda.