Both Puerto Rico and D.C. will naturally give Democrats the advantage and there is no other possible new state to balance it (Guam is the most Republican of territories, but its just too small in population to become a state).
Do you think it's plausible that Democrats could try to do this for at least one of those, with or without the GOP next time they get significant majorities?
Also, they could allow part of a state to secede from another. Eastern Washington and part of Colorado have expressed that desire if only in passing as they are rapidly losing a common interest with the significantly more liberal / populated portions of their states.
(Also, I'm a bit cautious to consider the filibuster as I have a feeling, even a hope, that it will be significantly altered to prevent obstruction in the near future)
Would open a can of worms. Also would end up with hilariously selfish moves, like rich areas trying to secede so they don't have to fund poor areas or political parties gerrymandering on an state scale.