The most likely of impossible scenarios, IMO, would be racial polarization dooming an unbudging GOP to control only a small part of the Midwest. Eventually, we would get Social Democrat and Christian Democrat factions.
Eh, even under huge racial polarization, the GOP would control much more than that. Here is how a hypothetical election would look like if suddenly race was the only thing that mattered, and Republicans only won states that are both at least 66% or more non-hispanic white and that voted for Trump in 2016:
Dem: 365
GOP: 173
While a crushing defeat, this map is roughly on par with what the GOP got in 2008, so it's not "permanent majority" territory either. Plus this map is kind of a "best case scenario" for Dems. An slightly more realistic map under intense racial polarization is this (adding tossups this time for places I am not 100% sure about):
Dem: 292
GOP: 199
Tossup: 54
So while Dems would have the advantage, it would not be permanent majority territory, not even close; though polarization also means that elections would be very inelastic.
Fun fact, under this scenario, there would be 22 states voting GOP, so they'd only need 4/9 tossup states to get the Senate.
