Richmond, Virginia - In the event of no Civil War, or at least Virginia not getting as smashed by it, Richmond would definitely be a greater city without having to be rebuilt and the continued status of West Virginia in the Commonwealth would likely prove a boon to industrialization, as the resources of the aforementioned state would be flowing to Richmond.
Big Stone Gap, Virginia - Change the Civil War (Averted or quicker ending) or remove Alexander Arthur, and such would've made available the capital needed to carry out the industrialization plans for the town. Such would've nerfed Middlesboro in Kentucky and perhaps Kingsport, Tennessee later on as Eastman Kodak might be more interested in developing its plant in the growing BSG. I personally don't think it could've become a Pittsburgh as was extolled, but a city of 50-80,000 with a strong industrial basis certainly seems possible. Such would've also kept the nearby coal towns relevant and large, first as sources of coal for iron/steel production, and then as suburbs as the town became a city. Add in UVA deciding to locate their affiliate campus here instead of Wise, and you'd definitely have the Virginian version of Johnson City, but with the industry of Kingsport thrown in.
Middlesboro, Kentucky - As kind of a reverse of the above, avert the 1890 Fire and have Barings not make such bad investments in Argentina, and Middlesboro could end up bigger than it did. If capital keeps flowing in, the railroad tunnel can be built and thus the city could become an important hub between Knoxville and Lexington, but the plans to turn it into a major industrial site were always going to fall short due to the poor quality of materials in the area. Overall, it could probably reach a size of 20-30,000.
Fort Blackmore, Virginia - I've never been able to confirm such, but a High School history teacher of mine stated that Eastman Kodak nearly built what became their Kingsport Plant in or near this tiny hamlet, but were stopped by some of the local tobacco growers. Had they failed, Fort Blackmore probably could've grown into a city of 30,000-40,000.
Johnson City, Tennessee - If you can somehow avert the Panic of 1893, Johnson City would continue to grow in size, and
might be able to turn itself into a second Knoxville.
Muscle Shoals, Alabama -
I'll let the town itself explain what nearly happened:
So kill off Norris, and you might be able to get a second Birmingham.
Mobile, Alabama - After the Civil War, trade began to shift away from the Mississippi and New Orleans lost some of its prominence in this regard. Had it got hit by a sufficiently strong Hurricane sometime in the 1870s-1890s, it's possible Mobile could've replaced it as the premier port on the Gulf, given its central location and proximity to the iron production sites at Birmingham.
Birmingham, Alabama - Avert the Civil War or have it won by the South, and Birmingham would've turned into every bit the rival of Pittsburgh that it was intended to be. Due to unfair Pro-Pittsburgh pricing rates forced into usage, Birmingham's ability sell was deeply undermined, and this was further compounded by the lack of sufficient regional capital to draw on due to the effects of the Civil War.
The Iron and Steel Industry of the Birmingham, Alabama, District by Langdon White (Economic Geography, Vol. 4, No. 4 (Oct., 1928), pp. 349-365) outlines the areas Birmingham would've easily been able to out-compete Pittsburgh in
at the minimum:
Duluth, Minnesota - U.S. Steel apparently narrowly chose expanding production in Pittsburgh over Duluth in 1911, a move which, if reversed, certainly would've brought more development.
Topeka, Kansas - Had it got the international airport over Kansas City, it could've went the Atlanta route of development.
Portland, Oregon - In the 1960s they tried to build a stadium to attract the Raiders and made a bid for the 1968 Olympics, but both ended up failing. Had they not, it would've obviously been a boon for the city.