By far the most likely actual kind of conflict is just an expansion of what we've already seen in the last decade or two: states or municipalities of one color

By far the most likely actual kind of conflict is just an expansion of what we've already seen in the last decade or two: states or municipalities of one color refusing to comply with the other color's legal mandates, and states forming color alliances against each other. For example, California has prohibited any official or municipality in the State doing any business or sending any representatives to the red States. None of the red States have done exactly that thing, but, for example, red border States are exploring programs that, for example, bus illegal migrants into blue States. This sort of behavior will probably escalate.