If there were a subway line built through downtown, I would think it would start at the Jefferson Circle in Goose Hollow, run down Jefferson, turn north on 4th, run up downtown to old town, then under the river and meet up with the light rail line that runs through the Banfield.
Though with that said, it would just be light rail trains running on that line and would probably me 4 car trains like the one in the photo.
I would also imagine that 1-2 car streetcar lines could also run on this same subway line to handle more streetcar lines through downtown.
I will admit it would be something cool to see in the city, but the cost of constructing a 3 miles subway tunnel through downtown, under the 405, under the Willamette, under the convention center probably would cost too much to ever really pencil out until there is a huge demand for more than 2 car light rail trains.
But if there were a downtown subway line built, that is what I would want to see. Plus at the intersection of Jefferson and 4th, there could be an interchange there to allow another tunnel to be constructed headed towards Powell and the Orange line for those two areas to be connected by streetcar and 4 car trains.