If the filibuster ends up being overturned to codify Roe v Wade, that would put Dems in the position of being able to pass a DC Statehood referendum, which the the people of DC would almost certainly vote in favor of, adding 2 extra Democratic senators.

Other Possibilities:

They could also finally pass a Puerto Rican statehood/commonwealth/independence referendum, although considering the more ambiguous nature of the political outcome, I think it's less of a shoe-in.

The fair representation act, could make gerrymandering far more difficult by requiring districts to have 3-5 members, and house & senate elections would have to use RCV, allowing 3rd parties to potentially get small numbers of seats in congress.