I actually agree that this situation would likely see Democrats start pushing for some various things to game the system, like splitting up California or trying to admit Puerto Rico as a state, for transparently partisan reasons.
At this point, I'm all for Democrats admitting Puerto Rico, and I'd hope, DC, for transparently partisan reasons, not necessarily for the political benefits but rather for the fact that partisan hackery is the best chance these regions have at getting proper representation. I'm not sure if I can say this for PR, but at least for DC the resistance to statehood or at least 2 Senators/1 Rep are themselves transparently partisan, so I feel no shame in my position.
I'm not sure about California - I do think it should probably be at least 2 states, but that's really up to them. I'd have to think that, regardless of what Democrats want to do in a shameless reach for power, something like splitting up CA wouldn't be possible without significant popular support.