Now that the USL has achieved provisional USSF sanction as a D2 league, they plan to launch a new D3 division in 2019,
according to Straus.
The USL contains some relatively "big", well-supported clubs (Cincinnati, Sacramento, Tampa Bay, Louisville), small clubs from small cities (Harrisburg, Reno), and a bunch of MLS2 teams that don't draw that well.
Eventually, it makes sense to have a D2 with the bigger clubs and D3 with the MLS2 teams and the smaller independents. Right now, I don't think the USL has enough clubs to have a D2 and a D3 division that each maintain the travel cost reductions that have partially underwritten the USL's recent success.
It will be interesting to see (1) how many potential D3 expansion groups the USL can scare up, (2) how many PDL groups may be interested in a step up, and (3) whether the NASL teams will finally call it quits and see the economic sense in consolidating.
There is always a lot of wishcasting around (2), but thus far zero PDL clubs have made the move up since travel was regionalized in (IIRC) 2015. Even
Kenn Tomasch has given up tracking PDL attendance, but in the most recent numbers (2014) you can see that there aren't a ton of candidates. In the NPSL, which is a similar league to the PDL run outside of the USL's umbrella, there are a handful of well-supported clubs such as those in Chattanooga, Detroit, Grand Rapids, and perhaps a few other places.
There are currently 38 professional clubs in the NASL (8) +USL (30), with Nashville confirmed to start in the USL next year. It's possible, though not certain, that Edmonton (NASL) and especially Ottawa (USL) could be out the door if the Canadian Premier League ever gets off the ground.
If the right owners and stadium situation can be found, there are still plenty of good markets for the taking.
A few of these would be rather unlikely candidates because of proximity to larger metro areas, but here are the largest MSAs without pro soccer
13. Inland Empire (it's decentralized, not particularly wealthy, and probably doesn't have great sponsorship potential, but it has >4.5m people, a fairly blank sports scene, and it's a long way to LA. I'd love to see someone give minor league soccer a whirl here.)
14. Detroit
17. San Diego
21. Baltimore
29. Las Vegas
31. Austin
32. Cleveland
37. Hampton Roads
38. Providence (well, unless you count the Revs)
39. Milwaukee
42. Memphis
46. New Orleans
47. Hartford
49. Birmingham
50. Buffalo
52. Grand Rapids (>1m, who knew?)
53. Tucson
54. Honolulu
56. Fresno
57. Bridgeport
58. Worcester
59. Omaha
60. Albuquerque
61. Greenville, SC
62. Bakersfield
63. Albany
64. Knoxville
65. New Haven
67. Oxnard/Ventura
68. El Paso
70. Baton Rouge
71. Columbia, SC
72. Dayton
Okay, I cut things off after 800k, but there are still long-standing (by US minor league standards) clubs in smaller markets like Charleston (760k) and Harrisburg (570k) as well as newer entrants who are at least surviving like Colorado Springs (710k) and Reno (460k)