Really hard to say on Texas. It has been so long since a Democrat won here at any level, even for statewide office that it is difficult to visualize it actually happening as the GOP seems to have a pretty solid 52% of the vote that will stick with them come heck or high water. Past winning Democratic maps like Lloyd Bentsen's in 1988 or Ann Richards' in 1990 (to say nothing of Jimmy Carter's in 1976) don't bear much resemblance to what a winning Democratic map would look like now. Like much of the South, Democrats lost Texas for good once the rural parts of the state abandoned them.
But if the major metro areas start voting more like major metro areas in the North do, then Texas will be competitive. There were definite signs of southern metro areas in general starting to behave more like northern ones this year (Dallas, Houston, San Antonio, Atlanta). A big reason why the South in general is so GOP is due to the fact that Southern cities and especially Southern suburbs are so Republican compared to cities in the rest of the US.
One reason for the reason for the South being so Republican is that the South is so much more rural than the rest of America. Florida and Virginia are much more urban than other traditionally-Southern states, which makes them much more Democratic. North Carolina is getting huge numbers of people from the Northeastern quadrant, and those voters are heavily Democrats.
The other is that the suburban areas are much newer, with relatively new infrastructure that does not have great costs for maintenance as one might expect in Illinois. Add to this, the housing stock is newer, so we aren't seeing 70-year-old tract houses being torn down for the building of apartment complexes with people who have little stake in political conservatism.
...Americans will be getting a hard lesson in civics as the populist Donald Trump gives way to the corporatist Donald Trump and goes from campaigning like Hugo Chavez to governing like Miklos Horthy. Lots of states will flip D in 2020 if they can if Americans see nothing more from Donald Trump than more hours of toil under harsher conditions for lower pay.
