What's the best candidate for the next NEC?
I might think that based on current frequencies and the regional population, it might by the Pacific Surfliner route. Not quite the NEC, which does have 5 metro areas whose population exceeds 1 million people (BOS, NYC, PHL, BAL, WAS), but the LA - San Diego Region sure has a lot of people. And the routes not only have the Amtrak intercity service, they also have commuter service. Plus, there is extensive rail (or light rail) connecting transit in both LA and San Diego. To truly NEC-ify the route, they need to double track and eliminate grade crossings. Then they might be able to run the trains faster.
The other NEC type corridor that might be built is Chicago - South Bend- Toledo - Cleveland, and Chicago - Detroit - Toledo Cleveland. If you also develop a Cleveland -Pittsburgh and Cleveland - Buffalo corridor, and if you ever get the Keystone West service (Harrisburg - Pittsburgh) running, you'd have corridor service overlapping the two main New York Chicago long-distance routes, which would allow for sharing of overhead costs and improving the financials for the Lake Shore Limited and and New York - Philadelphia - Chicago train. In fact, why not also a Washington - Pittsburgh Corridor, although going over the Sand Patch grade is very, very slow, and there aren't really any big cities in between Washington and Pittsburgh.
Most of these corridors might not serve enough population to justify more than 4-6 trains a day, but the Chicago-Cleveland service might generate enough business to justify hourly service.
Of course, the Southeast high speed rail (even if it's only "higher speed rail") would be a good candidate for an upgrade, and connecting Washington, Richmond and the Carolina Metropolises (Raleigh, Greensboro and Charlotte) might generate enough business to justify hourly service. These would also provide sharing of overhead costs for the Silver service and the Crescent.
There's certainly enough population in Texas along the I-35 corridor to justify hourly service between the Dallas Ft. Worth Metroplex and San Antonio, but the infrastructure is a little flaky (for example, the rail route bypasses Waco, one of the larger cities between Ft. Worth and Austin), and, of course, this is Texas. For that matter, a corridor connecting San Antonio and Houston, and Houston and the Metroplex would probably do well, too. Again, the only way suitable infrastructure will be built is with public funding, and the only way that's going to happen in Texas is when enough Californians move to Texas to change the political culture.
In general, I think the kind of future we should be aiming for is a whole bunch of corridor services (whether HSR or just "higher speed rail") with select long-distance connectors to form a national network. As far as the kind of attentive service that we seem to remember from back in the "good old days", I think that (1) maybe in reality it wasn't always as good as people remember, and (2) that kind of service is long gone from everything else in our society, so why should we expect it on Amtrak trains?