It will be interesting to see what a 51/50 majority can accomplish. A lot of it will come down to Manchin, who is less conservative then some think, but is a West Virginia populist.,
Packing the court is almost certainly a no go unless the court is egregious in striking down legislation or Biden's acts as executive
Medicare for All definitely a no go, Public option likely a no go but less certain
Green new deal is a no go, Manchin's not going to vote against coal.
DC/PR statehood is a possibility, but I wouldn't bet on it.
Shoring up/tinkering with obamacare is likely a yes
HR1 for the people act (voting rights, etc) is likely a yes
Tax reform is possible, but will likely be a neutered version. I wouldn't be surprised if the tax on 400k plus turned to 1 million plus. Also wouldn't be surprised to see SALT deductions return.
Increased COVID stimulus is all but guaranteed
Biden gets to pick his cabinet and court nominees
The biggest consideration will come down to filibuster overrides. I fully expect Mitch to go into full obstruction mode, in which case most of the above can't happen unless Manchin agrees to filibuster overrides. I don't think he will at first, but if Mitch is obstructing everything and the 2 years are slipping by he may change his mind.