Gobert - up-and-coming player before The Great War, winning Olympic Gold in 1912 and finalist at WHCC in 1913, losing to Tony Wilding; was Wimbledon finalist, and beat Wilding at British Covered Courts. Won French Covered Courts three times, British Covered Courts four times and European Covered Courts. Also French National Ch., beating Decugis in CR, the World Covrered Court Championship, an ILTF official world championship. Had some good wins against top players in 1920 and 21. The WHCC would have given him a Slam and perhaps pushed him close to the first rate, after five years lost at war.
Hunter - four-time Slam finalist (one Wimbledon, one US Pro). Tilden's doubles partner in late 1920s. Both US National finals went five sets.
Borotra - thought he had won the '27 Wimbledon final when his shot on match point grazed Cochet's racket (by nearly all accounts) but the umpire hesitated and asked Cochet who gave a non-reply and play resumed. The 1929 RG final vs. Lacoste was a taut battle, 6-8 in the fifth. I'd like to read about it. Either of these matches goes the other way and the relative positions among the three alters notably.
Hopman - he was the world's most famous coach (or Captain), but he was almost a champion.
Vines - He could not get it done at Davis Cup. The following year he lost to Perry in the Interzone Final.
Perry - he wins that match in Plainfield, and he takes a 30-29 lead in the WCS, instead of other way around. Vines won 32-29. Perry lost the match 7-9, 6-4, 3-6. Who knows?
Nusslein - Hans won Wembley, and every other major pro title. But a win at Wembley over Vines (whom he had beaten at US Pro) or, especially, the great Don Budge, might have raised him out of his semi-obscurity.
Puncec - In his mid-twenties, the Croatian was coming into his own just before WWII. In 1938-39 he posted a 19-1 Davis Cup singles record, won MC, semifinalist twice at Wimbledon and at RG; success on Riviera circuit. Primarily clay-courter, but Wimbledon shows he had promise on grass and a PSW would have made him world champ on cement. Returning after war, he was 33 years old and had lost the verve.
Petra - An elite clay-court title would have complemented Petra's Wimbledon triumph. WWII prisoner of war. In his early twenties, just before the war, compiled 6-1 Davis Cup record, won Coupe Noel and finalist at MC. In 1946 was semifinalist at RG, won Wimby, finalist at MC and 5-1 DC record. Made quarters at RG and Wimbledon in 1947. Thereafter barely played. Indications are he would have been a force during 1940-45.
Kovacs - after Rios, maybe the greatest talent to never win a Slam.
Budge - You'd think it wouldn't matter what, if anything Budge accomplished after the War. But Don badly wanted the '47 US Pro. He hoped by beating Riggs that he might get picked to tour with Jack Kramer. By some accounts he good have used the money. He had played Riggs near even in the '46 WCS and during their Euro-Africa tour in early '47. But Don and his wife were enjoying hanging out with Bing Crosby, Frank Sinatra and their gang, and he was drinking. Riggs was in better physical condition and outlasted the erstwhile god in five sets.
Bromwich - such a splendid player - and unorthodox! Six years lost to War, including grueling service in the Pacific infantry. Lost Wimbledon final after holding several match points. A shame . . . no shame on Bromwich. He did win a couple of Australians; seven New South Wales (Sydney); four Victorian (Melbourne); PSW; Queen's Club; beat Budge in '37 and Cramm at Australian '38; 22-7 DC singles, including heroics at '39 Challenge Round in US; 3-time SF at US Nationals; 13 men's doubles Slam titles. Another player who would have won some Slams singles titles during the war years.
Patty - with a victory in either of those two MC finals, Patty would have achieved winning the four jewels of clay-court tennis: RG, Italian, German/Madrid and MC (Clay-Court Grand Slam), a feat realized by only seven players, I believe (Drobny, Pietrangeli, Borg, Vilas, Kuerten, Nadal, Djokovic)
Drobny - Or a second Wimbledon, or third RG, either w/in grasp, would be significant enhancement to his rep.
Kramer - strangely less effective at big pro tournaments than WCS. Had Jake won this Wembley final, it might have damped down strong suspicions that he feared a WCS rematch with Gonzalez, and reaffirmed his No. 1 status.
Segura - won three US Pros in early 1950s. He remained a fiery competitor on the pro circuit, tied overall with Trabert and Hoad, better than 40 percent against Sedgman, and similar versus Kramer, and competitive with anyone. Won Aussie Pro and Masters Pro in late his 30s and was winning tournaments into his 40s. A Wembley title would have been nice.
Fraser - one heck of a athletic, power player for a brief period. One of the best serves in history. Back-to-back U.S. Nationals and beat Laver at 1960 Wimbledon final. DC singles record of 11-1 and hat-trick at '59 Challenge Round. Aussie 1960 would have given him his national championshp. Italian 1959 would have highlighted his versatility. Did win New South Wales twice and Victorian. Semifinalist at RG.