I think that US Nationals is still going to stick around in the Midwest region for the next several years, mainly for two reasons. For one, the midwest is the centermost region of the United States. When something is in the center, it usually tends to draw attention, almost as if people want to get to the "heart" of it (it may sound like weird ideology, but that's how I see it). The other reason is what Machamp the Champion said, which I really agree with. A location at the center of the country is "balanced out" in terms of distance. Both the East and West Coasts will have roughly the same distance to a location around the center of the United States, compared to having a location on either of the coasts, which would make it very inconvenient for the other. Thus, having the location around the center of the country (in this case, the Midwest) makes it convenient for both coasts in that one isn't too far for the other.
As for the specific location, I have feeling it will come down to two: Indianapolis and Chicago. However, Indianapolis seems to be the much higher candidate though. They have been hosting Nationals there for four years now, and its become clear why this is the case. It has a great venue overall. The convention center is large enough to accommodate all players in the event, and there are many great hotels that are connected to the facility thanks to the skywalks. In addition, there are plenty of places to eat nearby thanks to the nearby mall and the restaurants in the streets below.
If they chose not to host it in Indianapolis, I have a feeling that Chicago will be the next city they'll go to, and it too can be a great location for nationals. The city has three great convention centers to host the event: Navy Pier, McCormick Place, and the Rosemont Convention Center. If P!P wants to host the tournament in the heart of Chicago so that players can have a scenic view of both the Skyline and Lake Michigan, Navy Pier is one good location. They could also host nationals at the McCormick Place if they are looking for a gigantic venue overall, as the McCormick Place is a tremendously large place. The Rosement Convention Center is another great place to host nationals due to its location since its located right outside O'Hare airport. Just imagine. Upon arriving at the airport, you are already there! In addition to that, there are some nice hotels and a few restaurants close by.